To visit the Tom Ryan retrospective at the RHA Gallagher Gallery is a strange experience. You feel you have stepped back in time, back to another era of Irish history, to an Ireland certain of its nationalist, Catholic identity and anxious to emphasise it. Beside sombre portraits of religious, political and military figures, there are Irish historical paintings, genre subjects that are almost Victorian in feeling and more informal, personal portraits.

For many years, Ryan personified the RHA of which he was president from 1982 to 1992. He has aggressively espoused the cause of academic art and denounced and derided modern art, often with engaging and acerbic wit. In his eyes it was a clear-cut contest: the virtues of representational Irish art on the one hand, and degenerate, abstract modernism on the other. The conventional RHA view was that the values the academy stood for were being frozen out of the Irish artistic scene, which was ruled by a modernist clique.

Could Ryan's vociferous identification with the lost cause of academicism have been detrimental to his own place in 20th-century Irish art history? That is the line of argument put forward by Dr Phillip McEvansoneya in his admirably clear catalogue essay, which effectively puts the academic case. It is striking that, though the essay accompanies a major retrospective exhibition, most of it is given over to outlining this historical context. It doesn't so much deal with Ryan's art as provide a rationale for its relative neglect.

"Relative" is the right word. Ryan can rightly claim to be an exceptionally popular artist. He has received, and carried out in an impeccably professional manner, numerous portrait commissions, including some of the most prestigious ones going. He designed the one pound coin. His personal work, including landscape, genre and still life as well as figurative subjects, is sought and prized by a wide circle of collectors. So successful is he, in fact, that it is reasonable to ask why he, or McEvansoneya, should feel aggrieved about his position in Irish art?

From the academic point of view, the problem is that his art should, but does not, form the mainstream of artistic activity in the State. He believes in the role of the painter as a witness to his or her society. Hence his self-imposed task of chronicling pivotal moments of Irish history, including the Flight of the Earls, the Easter Rising and the first Dail. He has also treated Biblical subjects in much the same spirit. These paintings were made from the late 1950s on, but even at the time they were painted they had a distinctly anachronistic air. They do not so much perpetuate as pastiche a tradition. It's also true that large-scale dramatic composition is not Ryan's forte. He is more comfortable on a smaller scale and in a more subdued vein.

Such personal popularity and a measure of official approval, no matter how gratifying, are no substitutes for the centrality the RHA felt was its due. There is also the fact that, throughout much of his membership of the RHA, he was effectively part of an academy in exile. Without a headquarters and, more recently, with a shell of a building and no financial means of completing it, the academy was severely disadvantaged. It is greatly to Ryan's credit that during his presidency he oversaw the formidable task of making possible the completion of the mammoth Gallagher Gallery complex.

The irony is that by the time the Gallagher was eventually completed, the academy was a substantially changed organisation. Time had moved on. Academicians had aged, new artists were recruited to the ranks, including those who would previously have been regarded as antithetical to its ethos, and younger artists who worked in a representational vein.

McEvansoneya puts forward the view that Ryan's relative obscurity arose from a straightforward stylistic clash: academic, representational values versus, and then eclipsed by, new-fangled, experimental abstraction. Such an either/or, polarised view is so over-simplistic that it fatally distorts the issue. It doesn't take account of the debates within the gamut of modernist and postmodernist styles, and it conveniently ignores representational alternatives. For example, the Independent Artists group, formed in the 1960s, embodied an opposition to both mainstream international modernism and the academy. To hoover up all representational alternatives to modernism and label them "RHA" is very misleading.

Alternatives to academicism were not simply a matter of stylistic "experiment". They were shaped by many factors, within a much wider historical context. It could be argued that the academy withered because of its inability to develop an outmoded artistic idiom.

Its style was as historically determined as any other, as is abundantly clear in Ryan's work. One of his teachers was Sean Keating, and Ryan's painting is immersed in the values of the Free State style formed in the years following independence. This has been memorably characterised by Brian Fallon as the artistic equivalent of "those old iron letterboxes with green paint applied over the crest and initials of Victoria Regina and Eduardus Rex."

In some of Ryan's self-conscious history paintings, this shades over into a kind of patriotic social-realism, worthy and affirmative.

However, there is a sense in Ryan's work, that he would very much like to be describing a simpler, more straightforward Ireland, something approaching de Valera's ideal. A sense that, in fact, despite his self-assigned duty of witnessing his society and his times, he is describing not Ireland as is but as it might have been. Ryan is technically capable, but he is in no position to tetchily dismiss greater talents as he does. Some of his portraits are very good, such as that of Lieutenant General Gerard McMahon painted in 1998. But he is distinctly uneasy with large, complex compositions and his more ambitious subject paintings fail to convince. There is a streak of cloying sentimentality in his genre subjects. Some of his best work, though, is in landscape. He has made many fine, understated paintings that capture the distinctive character of Irish light and atmosphere. He is at his best in these works, when he is at his most informal and understated, and at his least bombastic - when he just gets on with the job.

Thomas Ryan: A Retrospective is at the RHA Gallagher Gallery until March 19th