His father was one of the founding officers of the elite Army Ranger Wing (ARW), Ireland's special operations force.

[4] Heaslip scored his side’s only try in the Heineken Cup semi-final in which Leinster were defeated 26–16 by the eventual champions Toulouse.

He was a member of the victorious Ireland team that won the 2009 Six Nations Championship and Grand Slam[5] He also was nominated for the IRB International Player of the Year for 2009.

[8] Heaslip was selected to captain Ireland against South Africa in the 2012 Autumn Tests, after Brian O'Driscoll, Paul O'Connell and Rory Best were all ruled out through injury.

[9][10] In January 2013, Heaslip was named by Declan Kidney as the new Ireland captain for the 2013 Six Nations Championship, replacing Brian O'Driscoll who had held the role since 2003.

[15][16] He had been due to play for Ireland against England in the 2017 Six Nations Championship on 18 March, but pulled out during the warm-up with what seemed like an innocuous injury at the time.