Reviewed:

Leonardo Drew, Royal Hibernian Academy, Dublin, until August 12th (01-6612558)

Felicity Clear, Rubicon Gallery, Dublin, until August 11th (01-6708055)

Leonardo Drew's vast work Number 77 looks as if it was designed for the Royal Hibernian Academy. It's a remarkably neat fit on one of the huge walls of the Gallagher Gallery, and as you come up the staircase the length of the room gives you a long approach run that yields successive layers of detail and drives home the scale of the thing.

Or, more accurately, things, because Number 77 is a grid-patterned assemblage of an extensive collection of urban detritus, of everything from bits of broken electrical appliances to fragments of animal skull.

Many thousands of such individual items are painstakingly glued to square wooden panels to make up a dense but orderly textural mass. Yet any detailed engagement with the work reveals that the promise of order contained in the grid is borne out not by the material that occupies it but in the way, perhaps, that the grid plans of American cities suggest an overriding logic and homogeneity that is undercut by the variable identities of the populations that inhabit them.

"Out of the crooked timber of humanity no straight thing was ever made," as Isaiah Berlin paraphrased Immanuel Kant.

Drew lives in Brooklyn - he was born in Tallahassee, Florida, in 1962 and originally went to New York to study design - and Number 77 invites associations with his adopted city on at least two, overlapping counts.

The first is the overall impression it creates of a teeming, colourful grid, as though we're looking down on the city; the second is an extraordinary eruption of massed lengths of scrap wood that form a wedge-shaped section of the overall composition. This jagged wedge looks like a cross between one of Frank Gehry's free-form architectural models and an impressionistic rendering of Manhattan from above. The much smaller sepia-toned drawings that augment the main piece resemble urban blocks, almost like brownstones.

Apart from a city grid, the large work also invites associations, if in a looser way, with circuit boards, microchips and other emblematic orderly patterns. Up close, the surface is rough, not only in the obvious sense of being thickly studded with a host of miscellaneous bits and pieces, but also in terms of treatment: it is scarred and scored, chopped up and burnt-looking and, overall, quite dark in appearance.

Jannis Kounellis and Anselm Kiefer are mentioned in a brief catalogue note as points of reference, the former, presumably, for his installations featuring concentrated masses of a few given materials, such as stone, and the latter, surely, for the sombre tonality, unprecedented textural effects and sheer monumentality of his work.

Kiefer has an extremely variable touch. When a work goes well it is energised by his audacity and his feeling for materials, and perhaps by a certain bravura. When it doesn't, its size can drag it down. One of his monsters, a diagrammatic account of the French royal line that is in the collection of the New York Guggenheim, is dead on its feet.

Drew's work could easily collapse under its inertia. It is, after all, a vast agglomeration of deadness, of things used up, broken and discarded, all presented with a scorched-earth brutality. Yet it doesn't collapse. In fact, something funny happens. As you negotiate its myriad details and labyrinthine spaces, it accumulates an energy derived from the histories of its constitutive objects.

Even though Drew is entirely non-committal and unsentimental in his use of his innumerable "finds", never presenting them as anything other than rubbish if that's the way we want to look at them, the overall work comes across as narrative-packed rather than, as you might expect, narrative-drained.

And it is a work that has to be encountered at first hand, just like the mixed-media hybrids of the Irish artist Paul Mosse.

If Drew pictures a world in terms of its residues, Felicity Clear, in her exhibition at the Rubicon Gallery, suggests we can only ever know the world in a fragmentary way. That, at least, seems the implication of her use of oblique, partial images and quotations.

The works are beautiful and distinctive, combining elements as technically diverse as fresco and print. That, at any rate, is how they look: as if she builds up a smooth plaster surface on board, paints onto it in rich, translucent layers of almost luminous intensity and also prints on it with a terrifically dense black ink.

The largest piece is composed of several panels and features a sort of rain of Clear's emblematic objects, transient things such as butterflies, pollen grains and bubbles.

But in the centre, across a gap between adjoining panels, is a tree that could well be something like Yggdrasil, the "world ash" of Norse mythology. The title is from Heraclitus, the natural philosopher of flux: "the most beautiful cosmos is a pile of things poured out at random."

The tree is a rare symbol of coherence and completeness for Clear and, lest we get carried away, she simultaneously undercuts it with Heraclitus's endorsement of randomness. The apparent image of completeness is, of course, just another fragment, another in the "pile of things" that the universe pours out.

Perhaps there is an implication that we can never deduce the nature of the whole from our fragmented perspectives.

Yet what emerges is not a debilitating pessimism but a spirited acknowledgement of the world's elusiveness in the context of our fleeting lives.