Heaslip was born in Tiberias, Israel, while his father, retired Brigadier General Richard Heaslip, was there on duty with UNIFIL. His father was one of the founding officers of the elite Army Ranger Wing (ARW), Ireland's special operations force. The youngest of four children, he has two brothers and a sister. Jamie and his family moved back to Ireland when he was still young, settling in the town of Naas where he lived until moving to Dublin aged 17. He attended Newbridge College, in County Kildare. In 2004, he starred in the U-21 World Cup, held in Scotland, where Ireland finished as runners-up to New Zealand. Following his performance, Heaslip was nominated for the IRB Under-21 World Player of the Year award.