Seán Keating, one of Ireland’s greatest 20th-century artists, painted it in 1923, probably in the Dáil, but it was never shown in a gallery.
It remained rolled up in his studio until it was discovered by art historian Dr Éimear O’Connor about four years ago.
It was restored and framed for the exhibition Seán Keating Contemporary Contexts, which was opened in Cork’s Crawford Gallery last night by Arts Minister Jimmy Deenihan.
“I had only read about the portrait and I just nearly died with delight when I discovered it — it was an art historian’s dream,” said Dr O’Connor.
Keating was born in Limerick in 1889 and studied art in Dublin and London under William Orpen.
He returned to Ireland in 1916 and painted some of the most iconic images of the War of Independence, the Civil War, the early industrialisation of Ireland, and the tragedy of emigration.
He also spent decades lobbying the government of the fledgling republic to better support the arts, before his death in 1977.
His missing Cosgrave is being exhibited along with several other of his political portraits and paintings, many of which have not been shown in public.
Among them is his portrait of Fr Michael O’Flanagan, the vice-president of Sinn Féin, and a portrait of Seán Moylan, the leader of the North Cork Brigade depicted by Keating in his famous Men of the South, which hangs on permanent display in the Crawford.
Moylan’s portrait, which was painted shortly after his release from prison, hasn’t been seen in public for more than half a century and is now hanging alongside the picture of his men.
Dr O’Connor, sister of singer Sinéad, has spent almost a decade researching for the exhibition which is being presented with the co-operation of the Keating family. “The curatorial focus is deliberately political,” she said.
Gallery director Peter Murray said as Ireland entered the “decade of commemorations”, the exhibition was timely because Keating was one of the few artists who addressed the politics of his time.