To say the National Gallery of Ireland's exhibition of prints by Roderic O'Conor is surprisingly good may sound like damning it with faint praise. But it is surprisingly good, in a low-key way. The low-key aspect is the disappointing thing about the show.

We know the elusive, irascible-looking O'Conor from his exceptionally bold and colourful paintings, and when a print show was mooted, it sounded as if there might be a treat in store, perhaps even the discovery of a hoard of previously unseen and colourful treasures. Unlikely, but tantalising.

The facts knock any such possibility on the head. O'Conor didn't make many prints, relatively speaking. And while it is good, and appropriate, to see a show devoted to the prints, most of what we see here was included in the O'Conor retrospective in the mid-1980s, and there have clearly been no startling discoveries or revelations since.

Some of his more enthusiastic advocates may disagree, but he seems to have approached printmaking as he approached so much else in his life: with great flair but in a frustratingly sporadic and erratic way. Isolated bursts of activity alternated with apparent indifference. It's almost as if he made prints out of curiosity, with the attitude that he'd get back and do it in earnest at some stage. He didn't get round to editioning those prints that he did make, so often they survive only in the form of a couple of artist's proofs. And colour doesn't feature at all.

As if to compensate, the installation, in the sepulchral gloom of the print room - the lighting, determined by the demands of conservation, seems even dimmer than usual - includes a handful of paintings related, directly or indirectly, to the prints.

There are, besides, several works by Armand Seguin, a friend of O'Conor and the artist who, although 10 years his junior, was his instructor in the medium.

It could be argued that the Seguins, like O'Conor's paintings, are there to beef up what would otherwise be a fairly thin show. Things are slightly more complicated than that, though, and in any case, the comparison is interesting and illuminating, particularly in demonstrating that the pupil quickly surpassed his teacher.

More, the show's curator, the O'Conor expert Roy Johnston, points out that because of the poor level of documentation and the informality of the duo's working arrangements, there has been an amount of cross-attribution between the prints of O'Conor and Seguin. This display incorporates a couple of revisions on earlier attributions, and rings true.

The current tally of prints by O'Conor is an exceedingly modest 43. He struck up a friendship with Seguin in Brittany, in 1891, and they remained friends. A Breton of peasant stock, Seguin died tragically young, in 1903. In the summer of 1893 they worked together making etchings based on the coastal landscape at Le Pouldu, close to Pont-Aven, a name forever associated with Paul Gauguin, who was, of course, the big influence on both artists.

Their facilities for printmaking were rudimentary but adequate. They worked on plates cut from sheets of roofing zinc, rather than copper, and there is evidence of trial and error in the rough, sometimes blemished appearance of O'Conor's work.

If the essence of print is craft, then O'Conor doesn't really hack it as a printmaker. His work is for the most part little more than adequate in terms of craft technique. Yet it has tremendous vibrancy and directness, and unless you're hung up on technique, you'd have to admit that his prints are very good works of art.

At their best they have the immediacy of drawing with a little something else, an added density that comes with the bite of the etched, inked line. And line is O'Conor's primary means.

He refines the elements of the coastal landscape to swirling, energetic patterns with brisk clusters of directional line.

Line both demarcates form and bulks it out, binding things as disparate as clouds and rocks, or trees and water, or light and shade into freely rendered but impressively cohesive compositions.

The prints relate clearly and directly to the extraordinary, conspicuously striped landscape paintings that he was producing at the time (the thesis identifying him as an antecedent of Sean Scully has yet to be written, but a plausible case could be made). But they are more than afterthoughts: they are fine works in their own right.

Later in 1893, O'Conor's father died and he returned to Ireland to settle his affairs. His father's death made him a landowner with matters such as the problems of tenant farmers to deal with.

He quickly decided not to go too far down that avenue, placed his business in the hands of a land agent and hightailed it back to France.

Thereafter he befriended Gauguin and went on being a painter. He did go back to print, with felicitous results, but not with serious or sustained engagement.

Of all the Irish artists who followed the well-worn trail to the Continent, he assimilated remarkably well and was up there with the best, in tune with developments, exhibiting regularly, acquainted with everyone.

But he didn't rely on his art for an income. If he had done, he might have been more inclined to pursue printmaking, to edition and try to sell his prints, for example.

And there is always the question of what he would have produced if he let himself loose on the Irish landscape. But for O'Conor, France was art, Ireland was duty.

The Prints of Roderic O'Connor are at the Print Gallery of the National Gallery of Ireland until August 26th