Younger brother of former Munster player Ultan, he played on an Ireland Schools side that included Irish captain Brian O’Driscoll.
O’Callaghan won an U19 World Cup medal in 1998, that side coached by Declan Kidney. Made U21 debut in Feb ’99 and since then has played at every international level. Featured in both the 2006 & 2008 Heineken Cup winning sides and this summer on his second trip with the British & Irish Lions captained the side to victory over the Southern Kings in Port Elizabeth.
In 2012 O’Callaghan joined the small group of Munster legends to represent the province over 200 times.
2013 saw Donncha feature strongly in the 6 Nations Tournament and again in the Autumn series of the same year. These were however his last appearances in green as Devin Toner becoming the preferred choice at Lock to partner Paul O’Connell.
2015 saw Donncha take on a new challenge. Following permission to be released early from his IRFU contract with Munster, O’Callaghan signed a two year contract with English Premiership side, the Worcestor Warriors.