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U.S. President Barack Obama was joking about his Irish connections at a St. Patrick's Day event on Thursday. (AP)

U.S. President Barack Obama found out years ago he had an Irish great-great-great-grandfather who fled the potato famine in 1850. He can now claim 28 living relatives who also descended from that Irishman, including a Vietnam veteran, a school nurse and a displeased Arizona Republican.

The president's newly identified relatives are revealed in a study released to The Associated Press by Ancestry.com, whose genealogists also traced descendants of 23 other Irish passengers on the ship that brought Falmouth Kearney to the United States when he was 19.

The survey allowed genealogists to further trace branches in Obama's family tree and others who arrived on that ship, the Marmion, on March 20, 1850.

According to the survey, the passengers' descendants live in Canada, Syria and throughout the United States. Among Obama's newly identified relatives is 83-year-old Dorma Lee Reese, of Tucson, Arizona.

"I'm not a Democrat, so I can't say I clapped," said Reese, a retired brain-imaging technologist. "I don't appreciate what he's done by any means, but I do appreciate that he holds that office."

Obama addressed his roots Thursday during a St. Patrick's Day luncheon at the Capitol.

"Now, speaking of ancestry, there has been some controversy about my own background," the president said. "Two years into my presidency, some are still bent on peddling rumours about my origins. So today I want to put all those rumours to rest. It is true my great-great-great-grandfather really was from Ireland."

After applause, he continued, "It's true. Moneygall, to be precise. I can't believe I have to keep pointing this out."

Kearney arrived with his brother-in-law William and his wife, Margaret Cleary. They were destined for Ohio, where Kearney's relative had left property in his name. Kearney married, had 10 children and later settled in Indiana, where he worked as a farmer.

Obama's mother, Ann Dunham, was a descendant of one of Kearney's daughters, Mary Ann Kearney, and Jacob William Dunham.