Tiernan is an equality campaigner and was the Political Director of the successful Irish referendum campaign for marriage equality which saw Ireland become the first country in the world to introduce marriage equality by a public vote. He subsequently became the Executive Director of The Equality Campaign, the successful Australian public vote campaign for marriage equality. They remain the only two countries in the world to have introduced marriage equality through a public vote.
He is also the Director of Equal Future, a global campaign working with LGBT faith groups to deliver equality. He served as Policy Director of GLEN – The Gay and Lesbian Equality Network in Ireland where he was responsible for designing and implementing the successful political strategy to pass civil unions legislation. He designed and launched KnowNow, Ireland's first free community based rapid HIV testing program. He served as Director of Organisation to the then Tánaiste (Deputy Prime Minister) of Ireland, Mary Coughlan as well as serving two terms as Mayor of his home town of Bundoran in Ireland.