He is from a Limerick hurling family, with his father and brother, both called Pat, having played for the county.
Heffernan was heavily involved with Labour Youth during his time in university, serving on the organisation's internal disciplinary committee.
A graduate of the University of Limerick, Heffernan is a primary school teacher by profession. Before becoming a full-time politician, Heffernan taught in a number of schools, including St Anne's in Whitechapel, London.
He was elected to the 24th Seanad in April 2011 on the Agricultural Panel for the Labour Party. He was previously a member of Limerick County Council from 2009–11 for the Kilmallock electoral area. He was an unsuccessful candidate at the 2011 general election for the Limerick constituency, polling 7,910 first preference votes (17.5%). He was also a candidate at 2007 general election for the Limerick West constituency.
He lost the parliamentary Labour Party whip in December 2012 after voting against the government on the Social Welfare Bill. The loss of the whip resulted in a falloff in Heffernan's Seanad voting attendance. In September 2013, it was reported that Heffernan attended only 23% of all votes in the Seanad, the second worst record.
In February 2015, he claimed he was forming a new political party in order to oppose Sinn Féin.
On 4 September 2015, the Social Democrats announced that Heffernan was one of their five election candidates.
It also appears that his profile on the social Democrats page has been removed.
Between July 31 and August 2, 2016, Heffernan was allegedly arrested 3 times for offences including spitting at Gardaí, and drunken behaviour while attending Indiependence Music & Arts Festival in Mitchelstown, Co Cork.