O'Connell was named as captain in the Ireland squad for the 2015 Six Nations Championship on 1 February 2015. He started in all three of Ireland's opening victories against Italy, France and England O'Connell won his 100th cap for Ireland in a 23–16 defeat to Wales on 14 March 2015, meaning Ireland could not obtain a grand slam. He started in Ireland's final match against Scotland on 21 March 2015, scoring his seventh international try in the 10-40 win and leading Ireland to a second Six Nations Championship, the first time Ireland have won back-to-back championships since 1948-49. O'Connell also won the Player of the Championship award for the 2015 Six Nations. He won the IRUPA Players' Player of the Year 2015 Award in May 2015. O'Connell scored a try in Munster's 50-27 win against Newport Gwent Dragons on 16 May 2015. He started in Munster's 13-31 2015 Pro12 Grand Final defeat against Glasgow Warriors on 30 May 2015.