Cairns first entered politics as an activist interested in LGBT rights, canvassing on behalf of David Norris during the 2011 Irish presidential election. Following a period living abroad in Malta, Cairns returned to Ireland and joined the Together for Yes campaign during the referendum on repealing the 8th amendment, the amendment which forbid abortion in Ireland. Cairn's experience during this time prompted her to join the Social Democrats in 2018. She was one of the founding members of the West Cork branch of the party.