Killian played for Limerick FC's underage teams from U16 to U19 before he signed a professional deal with the club in February 2015. Killian then first appeared in a first team squad when he was named on the bench for the July 2015 summer friendly against MK Dons and in 2016 he was brought in to Martin Russell's first team squad.

Killian's league debut came on 1 October 2016 when he came on against Cobh Ramblers as a late substitute but that would be his only league appearance that year. Killian signed a new deal with Galway in November 2020.