The statement confirms claims which were reported in The Independent last year that the picture, currently in the gallery's Leech exhibition, was almost certainly by a Sheffield-born painter and close contemporary, Stanley Royle (1888-1961).
Both the girl featured in it and the sun-dappled woodland setting exactly matched those in an almost-identical work by Royle, Spring Morning Among the Bluebells.
The National Gallery said The Goose Girl had been examined with a binocular microscope and this had shown clearly for the first time the letters "...ley" and "...le" from a Christian name and a surname, indicating Stanley Royle's signature.
Uncertainty about the work led to years of fierce arguments between art experts in Ireland and Britain, mostly conducted in the columns and letters pages of national newspapers. Among those convinced from an early stage that the attribution was incorrect was the art historian Dominic Milmo- Penny who pointed out that it appeared to belong to a whole series of Royle works with strong similarities to The Goose Girl.
But confusion over the picture's origins had been fuelled by the fact that it was in Leech's possession and used to hang in his studio.
It was still attributed to him at the time it was purchased by the Irish National Gallery in 1970. The Goose Girl is now thought to have been painted outside Sheffield around 1921, and not in Brittany as had first been thought. It had been thought to have been from a school of Irish Impressionists including Leech (1881-1968) and Roderick O'Connor who painted in France in the late 19th century.
The dispute reached a climax last year when Christie's in London held a sale of modern British and continental paintings, with Royle's Spring Morning Among the Bluebells reproduced in colour on the catalogue cover.
This brought home to many Irish observers for the first time the uncanny similarity. "`Spring Morning' appears to be a continuation of The Goose Girl, they are so similar," a Dublin art critic observed when the catalogue arrived in the post.
Raymond Keaveney, the Irish National Gallery's director, said the question of he authorship "has now been conclusively resolved". He said the gallery had always intended that the current Leech exhibition "would act as a catalyst to clarify Leech's relationship to this painting".