He became a member of Seanad Éireann in May 2011, as a senator for the Labour Panel.[5]The Irish Times described him in the Seanad as "a frequent, informed and often abrasive contributor across a wide range of areas, with a keen attention to the nuances of legislation".[6]
He unsuccessfully contested the Waterford constituency at the 2002, 2007 and 2011 general elections, before winning a seat in 2016. He was re-elected in 2020, when his 20,596 first preference votes amounted to 1.95 quotas,[4] and was the highest ever recorded in the constituency's history.[7]
Cullinane drew criticism on election night when a 30-second video uploaded to Twitter showed him ending his election victory speech with the phrases "up the Republic, Up the 'Ra and Tiocfaidh ár lá".[8][9][10] When questioned about the appropriateness of using these phrases associated with support for the IRA, Cullinane stated: "Yesterday was a very emotional day for me . . . It was a long count and obviously we were very excited and very proud of the vote we got yesterday in Waterford. The 30-second clip was part of a longer speech that I gave where I was reflecting back on the hunger strikes, reflecting back on the fact that Kevin Lynch stood in the Waterford constituency in 1981. He was someone who inspired me and inspired I think many republicans. The comments were made in that context."[11]
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