The work, Head of an Old Man, has been owned by the Dublin gallery since 1871.
It was thought to be a Rembrandt until Dutch experts declared it was a 19th-century fake with a false signature.
That judgement was queried more recently by Andrew O'Connor, the gallery's senior conservator. He has established that the painting dates back to about 1650, and is consistent with Rembrandt's style.
Mr O'Connor said: "I always liked the work and my faith in it wavered in the face of all the experts. But I always felt it deserved reassessment, and now I have been able to do that. Cleaning tests and examination of the original paint pretty well convinced me.
"I also showed it to the head of the Rembrandt Research Project in Amsterdam, who endorsed my feeling.
"I would be hoping for confirmation within the next year, but these things always take a long time. And you are never able to say without any doubt that something is genuine."
Mr O'Connor was reluctant to put a price on the painting, if it is finally declared a Rembrandt. "For us, paintings are not related to commercial value, and this one is certainly not for sale," he said.
The last old master confirmed at the gallery, Caravaggio's The Taking of Christ, was valued at pounds 26m.