He is the son of actor Denis O’Dea and actress Siobán Mckenna. Donnacha was close to win a bracelet at the 1983 World Series of Poker in the $1.000 Limit Hold’em event. He got to the final table at the WSOP main event, but he got sixth being defeated by Tom McEvoy, the winner.

On 1991, Donnacha O’Dea got ninth in the WSOP main event. He won money at the 1992 World Poker Series main event (getting the place number 32), on 1994 place number 27, on 1995 place number 25 and on 2007 he got to the place number 171.

On 1998, O’Dea won his first WSOP bracelet at the Pot-Limit Omaha event beating double champion Johnny Chan. Donnacha appeared for the first time at the TV show Late Night Poker in the series 4, finishing in fifth place in the poker won by Robin Keston. In the series 6, he got fourth at the finals.

On 2004, he won the poker tournament Poker Million defeating Dave Ulliot in the heads-up event. O’Dea was the first European poker bonus player at the European Poker Players Hall of Fame. His winnings in tournaments by 2009 were over 1 million dollars.