Brennan scored his debut goals for his new club in United Park versus his hometown club, netting a freekick and a penalty in a 2–1 win for Bohs. He continued his good form netting 5 times in May 2008 and this did not go unnoticed as he was awarded Soccer Writers' Association of Ireland player of the month for May. Killian's goals helped Bohs blow away the competition as they marched to the 2008 League of Ireland title and his form was rewarded when he was voted on to the League of Ireland Premier Division Team for 2008, receiving a staggering 86% of the votes for the left midfield position.