The Bridesmaids star was looking to buy a house in his homeland when he found a dilapidated castle in need of renovation, and he was intrigued when he found out the building's name.
The actor then began researching his family history to find out if he had links to O'Dowd Castle.
He tells Britain's The Independent. "Weirdly, I... started looking into my family history in the last two years because I wanted to buy a castle. It was a basic two-up, two-down Irish castle with no roof - it was a wreck. Irish castles aren't that fancy. The only reason I was interested in it was because it was called O'Dowd Castle. But in the end I didn't buy it."
However, O'Dowd reveals he discovered a shocking family secret while he was researching his ancestors: "I found out that my great-grandfather was a bigamist. He was a travelling salesman and had a second family in Birmingham (England). Apparently that strand of the family gave birth to a George O'Dowd."
The actor is now convinced he is related to British singer Boy George, whose real name is George O'Dowd, and he has even shown his findings to the pop star, adding, "I've mentioned it to Boy George and he's quite into the idea that we might be cousins."