"I had no idea! I thought I was Scottish and Welsh. It turns out my parents are just full of s**t, I guess. But now I'm Irish," she said on the chat show.
She jokingly told Conan - who himself claims to be 99.8% Irish - that it all makes sense. "I'm a really good drinker. I love St. Patrick's Day. Potatoes are delicious. I'm looking forward to meeting all my cousins," she quipped.
Watch the clip of Reese talking about her Irish heritage: