Retired celebrity athletes from other sports making speeches to teams, a Ryder Cup tradition, brought early support to their sides at Hazeltine ahead of this week's 41st edition of the biennial men's golf matches.
Phelps, who spoke to the American squad Monday night, won a record 23 gold medals plus three silvers and two bronzes over five Olympic trips, including five gold medals at last month's Rio de Janeiro Games. The 31-year-old won a record eight golds at the 2008 Beijing Olympics.
O'Connell, a 36-year-old former lock, played 108 international matches for Ireland, captaining his national side as well as Munster and the British and Irish Lions in 2009 and 2013.
"Everyone I've ever spoken to who has heard Paul address teams says he's seriously inspirational," Europe captain Darren Clarke said. "I feel very fortunate and privleged that he's there to join us."
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Lyrics tell of Ireland and Northern Ireland players coming together as one team -- "Ireland, Ireland, together standing tall, shoulder to shoulder, we'll answer Ireland's call."
It's Clarke's call that O'Connell answered for a Tuesday night talk and the rugby union star will try to help Europe win on the road to capture an unprecedented fourth consecutive victory over the Americans.
"Shoulder to shoulder, it resonated with me," Clarke said. "It was something I pursued through my captaincy. There's no finer example of shoulder to shoulder than Paul O'Connell. He has legendary status at home and in terms of leading the British Lions, who always play away from home.
O'Connell brought Munster to the European title in 2006 and 2008 plus Celtic League crowns in 2003, 2009 and 2011. He sparked Ireland to Six Nations titles in 2009, 2014 and 2015 and had three tours of duty with the British and Irish Lions, winning their Southern Hemisphere tour in 2013.
Clarke has made certain that O'Connell's background is not lost on such players as Spain's Rafael Cabrera Bello or Belgium's Thomas Pieters from nations where rugby is not as huge as it is in Britain and Ireland.