Although using his trademark methodology - starting from digital and photographic tools and source material, and finishing with Old Master techniques on canvas - these new, seductive hyperrealist images mark a radical change of direction in a near 40-year career in which Helnwein has posited provocative figurative imagery in galleries, public spaces and even the media. These are usually expressions of - and lightning conductors for - emotion, shock, outrage and even humour.

But these landscapes also mark a return to his earliest artistic inspiration. While at the Academy of Fine Art in Vienna, he and some fellow students (including Manfred Deix and Robert Schoeller) were gobsmacked by a full-blown museum exhibition of German Romanticism and Realism. Somehow freed from the taint of Nazism, these paintings appeared, as Helnwein puts it, "like light falling into a dungeon". In response, they began trekking into the mountains of Austria and Southern Germany to sketch and paint en plein air. They immersed themselves in the literature of the Romantic period - writers like Heine and Eichendorf - in anachronistic protest against the shrine to Abstract Expressionism which the Academy had become.

Helnwein's meticulous Irish landscapes, which are the cornerstone of this Crawford show, are unashamedly aesthetic: gorgeous confections of pure, delicious spectacle. The typically epic but not inhuman scale imitates the subject matter. The tonal realism will make people go "Wow, are they paintings?" - thanks to the photorealist finish which seems free of the foibles of the human hand. Helnwein works with very small brushes - highlighting and subtly magnifying here, muting colours or creating shadows there; pushing some paintings towards momentary sleights of impressionism; and others towards seamless, burnished hyperreality.

Because each landscape starts life as a huge composite image worked up from high-defnition photographs, they appear less perceptually or psychologically embued than photographic in nature. But these are far from "literal" transcriptions of these views, which Helnwein has deliberately depopulated. Nothing here is accidental or entirely an artefact of a camera lens.

The bird's eye view suggests a kind of superhuman vision which can simultaneously take in the entire view with breath-taking clarity, like some bionic eagle. In fact, even to absorb the larger, panoramic paintings involves a combination of standing several feet back, and then moving closely along the canvas, squinting in at the limits of resolution.

Certainly, the photographic starting-point has its merits, considering the rapidly changing Atlantic weather and Irish light which can transmogrify a scene within seconds. Mountains can shimmer between colours while you glance at your watch. The soft, gauzy haze of easterly pollutants mute colours and blur contours. Vapour-saturated air produces dramatic lensing effects; while transitional weathers create aching tensions between, say, rain-pregnant cloud, and sudden sunlight which whack up a hillscape, say, into minute legiblility. Such a view is Irish Landscape IV (County Waterford), 2002-4, a light show which Helnwein renders like a glowering tone poem between the roving pools of sunlight and the squid-ink sky. Some fields are laden with gloom under an infestation of aphid-like sheep, while others are laser-read by miraculous searchbeams, radiating through the menacing raincloud like a hint of the divine.

But this is almost a sketch when set against the larger panoramas. Helnwein's first venture into the Irish landscape, the Nire Valley painting, was produced by photographing from a number of lofty positions on one side of the valley. In a peculiar example of the way he recycles images, Helnwein is exhibiting two paintings of this scene: the first Irish landscape he ever painted (courtesy of its owner, Hollywood actor, Jason Lee), and a new, larger version, five metres wide by one metre high.

Both are awesome views of a glacial landscape: the cloudshadow meandering over the undulating terrain; the once-forested valley denuded by millennia of human occupation, while the incisors of sheep have given it a added, botanical buzzcut. The naked rock breaks through on the dramatic upper slopes. To the left sits a corrie, a gigantic hole scooped out by an immense, rotating, rock-studded glacier, the corrie's walls rising up like an unfinished Mount Rushmore.

It's an implacable piece, with "nature" captured model-size: from the two snowy lambs in the left foreground, to the snaking, tiny lines of old stone walls; through scrubby, hawthorn hedgerows around more unkempt, boggy fields, to where trees, reduced to soughing broccoli, delimit a sunlit meadow beside the life-giving oxter of a watercourse, and back up the scrubby slope to where zigzag field-lines direct the eye to a highly Romantic conifer copse, etched against the blue-misted hills.

Another dramatic piece is Helnwein's sublime, seven-metre-wide take on an evening sky as seen from the roof of Tullamaine Castle in Wexford. It's another meticulously crafted representation of the commonplace wonders of a protean Irish sky: a symphony of light in constant interplay with the ragged aerial irrigation systems of the cloudscapes; vast sheets of broken stratocumulous advancing, like a host of vague snub-nosed zeppelins, over spinneys of mature demesne trees - from the gathering gloom in the south-east to the Turnesque furnace in the west.

Again, the drinkable detail: the wind-flung, scarcely visible specks of birds against the instantly recognisable Irish sky-palette - the mauvy-purple of the darkening clouds, their flanks tinged golden by the sinking sun. Through them, the azure sky peeks through, bearing no signs of rain, to every shepherd's delight; while faded Virgin Mary blues recede into smoky yellows in the distance. To the right, the sun sinks over darkening hedgerows and telegraph poles - and off into the far distance where the palest peaks melt into the false horizons of clouds.

A gentler Irish panorama is the view from the roof of Helnwein's neighbours in the rich Munster countryside, Lord Andrew and Lady Madeleine Lloyd Webber. It's another eagle's eye view over the managed, manorial landscape, with its pleasurably fizzing expanses of slightly surging woodland. Again, slanty, evening sunlight gilds the tips of the tree-canopy and warms the horse paddock over on the left; with its tractor ruts etched in high relief. Above that, more grazing and hay fields climb into Coillte plantations and blanket bog on the rising shoulder of Sliabh na mBan, the sacred mountain associated with myths of Finn McCool and his Fianna heroes. To the right, pastoral landscape recedes off into more blue-remembered hills. Odldy, Helnwein has maintained the presence of a farm house, nestling amongst the trees, and even - as though distantly heralding the approach of Thomas the Tank Engine - the white smoke of a local factory.

Otherwise, these are highly idealised views, with ne'er an aeroplane or fragmenting jettrail to interrupt the skies. They breathe a sense of a year well advanced, after the effusive growth and floral displays of May. Sunset seems to beckon, and with it, the sense of Romanticism Helnwein invokes, that behind the beauty, these are landscapes in long and tragic decline. Helnwein has a point. Having been spared the Industrial Revolution that made vast conurbations of England and Europe, Irish landscapes over the last decade have seen unprecedented development which is destroying 1,000 kilometers of hedgerow a year. As I write, the government is introducing legislation to allow them steamroll a density of archaeological treasures unrivalled in Europe. At a time when the population is rising for the first time since the Famine, the general view seems to be that we Irish are so spoilt with landscape, we can wipe our arses with it. The metaphor is not entirely inappropriate. Recently in the Silvermines Mountains in Tipperary, faced with a toxic swamp from the old mine outflow, somebody has had the bright idea of killing two birds with one stone - and pumping raw sewage in on top of it.

Helnwein's stated affinity is with the Romantic Age which began in the late 18th century, when the Renaissance and Classicism gave way to materialism and industrialism. In the air at the time were the stirrings of a resistance to Enlightenment rationalism. Irishman Edmund Burke had influentially dusted down the idea of "the Sublime" in aesthetic experience - in terms of the stronger emotions (including terror) and more irrational elements of art which were at odds with neoclassic order and harmony. It became a defining notion for the Romantic Age, and nowhere more so than in the German-speaking world, where all the high art forms were in full efflorescence.

German Romanticism is difficult to define, and is perhaps best described as a powerful, broad frequency of the imagination which has informed German art, culture and national identity ever since its inception. The emphasis is on a sense of powerful longing, emotional grandeur, expansive imagination, high idealism, bucking convention, a mystical unity with Nature and the divine spirit contained with it - all emblemised by the Blaue Blume, the motif of yearning evoked by Baron von Hardenberg (1772—1801) - aka Novalis, the “prophet of Romanticism". But such Romantic fervour always contained the seeds of virile Germanic nationalism. After all, the 19th century's dawn saw the Napoleanic humiliation, when the German Holy Roman Empire collapsed. The Germans looked simultaneously forwards to the end of French occupation, and backwards to the late mediaeval period when Germany had been united into a powerful indigenous culture. Radical Romantic patriots took to wearing German costumes from Dürer's time - the altdeutsch cape and floppy hat - a practice which was banned after the Congress of Vienna (1815) by conservative authorities.

But ancient national roots were breaking out of the soil all over Europe. While the Brothers Grimm were collecting German folklore and legends of the German Wald - and translating Thomas Crofton Croker's Irish fairy tales! - Edward Bunting was notating the dying strains of the old Gaelic harpists - which tunes were sweetend and set to lyrics by Irishman Thomas Moore (1779-1852) in his ten-volume, Irish Melodies which was hugely popular across Europe, and left Irish ears echoing with the immortal parlour warble of The Harp That Once Through Tara’s Halls and Believe Me, If All Those Endearing Young Charms.. (Incidentally, part of Moore's 1817 poem, "Lalla Rookh" - based on a Persian folk-tale - provided the libretto for Robert Schumann's neglected Oratorio "Das Paradies und Die Peri" - a Düsseldorfer Symphoniker production of which Helnwein recently designed).

Meanwhile, the "nationalisation" of the Irish landscape began, paradoxically, with the Ordnance Survey mapping project, and its Memoirs. Scholars and topographical artists, including the polymath George Petrie, recorded ancient monuments and fused them to old Gaelic literature. Petrie - a prolific landscape artist recently lionised by the Crawford - was a consummately Romantic painter, whose Irish views groan with awe-inspiring emotion: the reeling cliffs at Dun Aengus Fort; the terrifying shafts of sunlight on the lake at Gougane Barra; the pilgrims and gravestones like staggering ants beneath the ivied, crow-thronged stonker of Clonmacnoise's ruined tower.

But however much blood-and-soil nationalism became entangled in the late dawning of Irish Romanticism, German Romanticism remains radioactive, thanks to its full-scale absorption into Nazi ideology: the cautionary, "pessimistic" vision of Schopenhauer's The World as Will and Idea: Nietsche's idea of superhumanity in Also Sprach Zarathustra; and of course Wagner's musical absolutism and virulent anti-Semitism - particularly in light of the pan-Germanism which continues to wax and wane, particularly since German reunification in 1989.

Helnwein drew my attention to his key artists from the "Biedermeier" period (roughly 1815-1848) which, in a sense, is the antithesis of Romanticism in that it is associated with confined spaces, bourgeois familial bliss, sturdy rustic or small-town values. It also signals the rise of Austrian and Bavarian art: the magical child-paintings of Peter Fendi and Josef Danhauser; the astonishing, harshly lit, virtual photorealism of Ferdinand Georg Waldmüller; and the great humorist-fantasist, Carl Spitzweg's parade of comic eccentrics and pompous burghers.

But Helnwin's real touchstone is the great Romantic painter, Caspar David Friedrich (-1840), whose majestic, symbolist compositions of forests and mountains which, say, unify Nordic tree worship and the living Crucifix as a sprouting evergreen fir - are the visual equivalent of a Wagnerian horn-section.

In Untitled (After Casper David Friedrich) (1998), Helnwein, in Warholian fashion, boldly appropriates - and pays deep, cinematic, blue-black homage to - Friedrich's charged, nature-melodramatic painting, The Polar Sea (1824). Helnwein quoted this painting before in his triptych, The Silent Glow of the Avant Garde I (1986), flanked by two self-portraits like guardian angels: head-bandaged and doused in red paint, as though blindly, bloodily scanning the skies for enemy spitfires.

This latest version magnifes the painting hugely in a moody, spectral, sugary-icy blue monochrome which heightens the natural contrasts, and the sense of drama thanks to Helnwein's lighting effects, which impose a peculiar solitude and perspective. However, Friedrich's original is all there - Nature as a constant, infinite, momentous turnover of forces, the ice-sheets bludgeoning into each other in great planar cross-collisions of elemental power, splintering into lethal jags and patterns which emulate tectonic processes in a constant universal pattern of pulverising flux. You could nearly miss the tragic little galleon, like a child's toy tossed from the grinding tumult of splintering Matterhorns - a particularly Germanic-Romantic celebration of humanity dashed against the intensity and dispassionate cruelty of nature in all her savage pomp.

Although based on close studies of ice during the vicious winter of 1820-1, this icescape is not entirely imaginary. As a child, Friedrich witnessed, during an ice-skating excursion on the Baltic Sea, the drowning of his brother Christopher, who died while saving him. But no doubt, it is a more generalised allegory of the hubris of human endeavour, or our mortal slouch towards the grave.Certainly, Friedrich was deeply religious (although not above the odd amorous image). Goethe, inspired by Englishman Luke Howard's classification of clouds, made the mistake of asking Friedrich to illustrate a meteorological tract of his - Friedrich was outraged at such sacrilege.

Interestingly, between 1820 and 1824, Dresden, where Friedrich lived, had been treated to three great panoramic paintings of North Polar Expeditions. These were the populist spectacles of their time: giant canvases in a circle up to 100 metres in circumference and twenty metres high and shown in custom-built rotundas, with a viewing platform at the centre, with objects between the canvas and the viewers. Friedrich planned to paint one himself around 1810, an idea he later dropped.