Jennings impressed for Leinster in the 2004–05 Heineken Cup when Keith Gleeson was injured, however Gleeson returned from injury for the quarter final with Leicester Tigers, having had no previous game time. Leicester had previously identified Jennings as a threat. After the game which Leicester won 29–13, Leicester's retiring openside Neil Back approached Jennings and offered to open contract negotiations. He signed for Leicester for the 2006 season, along with fellow Leinster forward Leo Cullen winning the English Premiership in 2007. He re-signed for Leinster on a three-year contract for the start of the 2007/8 season winning the Magners League in 2008 and European Cup in 2009.  In October 2009 he was banned for twelve weeks for making contact with the eye area of Nick Kennedy of London Irish in the first round of the Heineken Cup. On February 19, 2015, Jennings announced he would retire at the end of the 2014-15 season.
Jennings's international ambitions were hampered largely due to a surplus of quality Irish back row forwards during his playing career. He was named in the Irish squad for the Summer tour of Argentina and earned his first cap on June 2, 2007. Jennings was called into the Ireland A side that was defeated by England Saxons on 1 February 2008. Jennings achieved 13 test caps throughout his career, his last coming against Scotland in the 2012 Six Nations.