The BBC/HBO production has been written by Spinal Tap and A Mighty Wind creator Christopher Guest.
"I'm terribly excited and monumentally under- qualified to work on an improvised show with Chris Guest," the Bridesmaids star said.
"I call him Chris because we are friends. He calls me George. I don't know why."
Guest has joked that he has been a fan of Chris for years. "Chris O'Dowd has been on my radar ever since he was a child actor in Wales," he said.
IT Crowd actor Chris has already started filming the eight-part comedy series. Shot in both the UK and Los Angeles, the show will air next spring and is being billed as a highlight of the comedy calendar for both HBO and the BBC.
Family Tree will tell the story of loser Tom Chadwick (O'Dowd), a 30-year-old who has just lost his job and his girlfriend. To get over being dumped, Tom sets off on an around-the-world adventure.
O'Dowd is becoming something of a regular on US TV sets; next month Chris will make his debut on hit US series Girls.
Chris plays part-time DJ Thomas-John and spent his spare time on set teaching the American actresses Irish slang words. "I told them some Irish slang. Just the common things," he said. "But they're not broadcastable."