It is a great pity that all the attention paid to Jack B Yeats has meant the considerable talents of his father are still so little appreciated. Even during his own lifetime, John B Yeats's talents were undervalued, not least by himself. William Murphy's excellent 1978 biography of the artist is appropriately titled Prodigal Father; Yeats consistently failed to live up to the expectations of his admirers and invariably disappointed those who tried to help him.

Born in Co Down in 1839, the son of a Church of Ireland rector, John B Yeats had as much charm as talent but sadly an insufficient amount of discipline. Few artists have been offered as many opportunities as he was, and even fewer have managed to squander their chances with as much grace. In 1901, for example, Sarah Purser organised an exhibition for Yeats and Nathaniel Hone in St Stephen's Green. The latter artist did not need the attention or the work which might follow, but Yeats, then in his sixties, was still struggling to gain commissions, although no one ever doubted his abilities.

A review of the show in the Daily Express described Yeats's oil of the nationalist John O'Leary as "probably the finest portrait painted by an Irish artist in recent years". Henry Lamb said he found in the Irish artist qualities similar to those found in the work of Courbet and Titian. Thomas Bodkin commented that not only was Yeats strikingly accomplished in the technique of painting, but that his approach was intellectual rather than sensuous. According to Bodkin, some of Yeats's male portraits "have an air of mingled dignity and intimacy that no other painter of modern times surpasses". The show arranged by Sarah Purser led to the most substantial commission Yeats was ever to be offered. Among those who visited the exhibition was Hugh Lane, then working as a dealer in old masters in London, while on a visit to Ireland. So impressed was Lane with what he saw that he not only bought several pictures, but also invited Yeats to paint a series of portraits of distinguished Irishmen as the nucleus of a national portrait gallery in this country. Lane's aunt, Lady Gregory, had already asked the artist to undertake a similar series, but of drawings rather than paintings and for her own possession. Both acts of patronage seem to have been motivated by a mixture of admiration for the artist and awareness that he needed to earn money. The problem was that Yeats loved to talk as much as to paint. He was a brilliant and entertaining conversationalist and even after Lane had set him up in a studio on St Stephen's Green, more time was spent discussing what he intended to do than in executing it.

Admittedly Lane, who like any other good dealer liked to buy pictures for a bargain, paid Yeats very little for his work. Still, it must have been dispiriting to have heard from the artist that he was enjoying himself immensely with one sitter; "we are now I think great friends, enjoying each other's society, as if there was no portrait between us". All too often, this last point was quite literally true.

Nonetheless, Yeats did finish a number of pictures depicting, among others, Horace Plunkett, Edward Dowden, Synge, Willie Fay and his son, W B Yeats. The last of these was regularly tapped for money whenever his father's funds ran low. So too was the American lawyer and art collector John Quinn, who became Yeats's most important patron after the artist moved to New York in late 1907. He remained in the United States until his death in 1922, when an oil portrait of Quinn begun 11 years earlier was still unfinished.

John B Yeats's career must be regarded as something of a failure for which no one other than himself can be held responsible. But the portraits he did produce, a number of them on permanent exhibition in the National Gallery of Ireland and the foyer of the Abbey Theatre, are wonderfully vivid affairs, full of psychological insight and delight in his subjects.

Most portraitists, when given an official commission, tend to produce their most uninspired work, reserving spontaneity for personally selected sitters. This was never the case with Yeats.

Perhaps because his output was so low, despite living such a long life, examples of his work do not come onto the market very often. Still, it is extraordinary that the record for one of John B Yeats's paintings is a mere £26,000, the price paid at a de Vere's auction in 1998 for a gouache of the Abbey actress Maire Nic Shiubhlaigh.