Noted for his wing play and ability to fire in accurate crosses, Brennan was signed by Peterborough United in his teenage years. In 2004, he was capped several times for the Republic of Ireland under-19s, before moving back to play in the League of Ireland. He was signed by John Gill for Dublin City, however, less than a year later, newly appointed boss Roddy Collins' 'purge' of the club meant that Brennan was deemed surplus to requirements. Immediately snapped up by Stephen Kenny for Derry City for the 2004 season, the player was a revelation and made the left wing spot at The Brandywell his own after having made his debut in a home game against St. Patrick's Athletic on 23 September 2004. He has also been deployed to fill in at left full-back on occasion. In the 2006 season he played in their UEFA Cup run in wins over IFK Göteborg and Gretna FC and helped Derry to the FAI Cup and League of Ireland Cup. It is this abundant talent that attracted the attention of his hometown club, Drogheda United who declared their interest in Brennan following the end of the 2006 season. However, it was Bohs who captured his signature, signing the winger on a free transfer.
Bohemians
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. He scored against Rhyl F.C. in the 2008 UEFA Intertoto Cup. Brennan helped Bohs to the 2009 League of Ireland Cup against Waterford United by scoring a freekick and a penalty as Bohs went on to win the match 3–1. He then scored again on 20 October as Bohs beat St. Patrick's Athletic to put them top of the table and that's where they finished as Bohemians won their first ever back-to-back league wins. Despite winning the Setanta Sports Cup, Bohemians had a disappointing 2010 season where the club lost their league title on goal difference and failed to make an impact in Europe where they made an embarrassing exit to Welsh side The New Saints. Killian did however net the winner in the first leg of that European tie. Due to financial difficulties Brennan was released when his contract expired at the end of the 2010 season. However, he was then resigned to play for Bohs for the 2011 season. On 27 July 2011, Brennan was a part of the starting XI for Coventry City's pre-season game against local non-league side Leamington. It is understood Brennan was on a trial basis at the club. On 14 January 2012 Brennan signed for League of Ireland ChampionsShamrock Rovers. After a poor season by Rovers which saw them finish with no silverware and outside of the European spots, Brennan decided to move on after just one season.
On 19 November, Brennan signed for St Patrick's Athletic. The signing was announced by the club's Head of Player Recruitment, Dave Campbell on the club's own radio show, The All Saints Show, on West Dublin Access Radio. His signing was officially announced on the club's website the next day. Brennan started off life at his new club brilliantly, getting the Man of the Match award in a 0–0 draw at home to Glentoran live on Setanta Sports. His performances away to Glentoran and at home to Drogheda United in the league opener stood out from the rest with his excellent ability to begin attacks. Brennan's first goal for the Saints came when he scored the winner from a penalty away to Limerick at Thomond Park on 30 March 2013. Brennan's second goal for the club came on 19 April 2913 when Pats faced Sligo Rovers at Richmond Park live on RTÉ, he chipped a cheeky penalty down the centre in the 89th minute as Pats won 2–0 and Brennan was awarded the Man of the Match award. On his return to his hometown, he scored a 92nd-minute winner from the penalty spot in a 2–1 win over Drogheda United on 6 May 2013. Brennan came back to haunt his old club Bohemian, when he blasted a penalty into the top corner to seal a 2–0 win for Pats putting them top of the league on 24 May 2013 at Dalymount Park.
Personal
Killian has three brothers all who play in the League of Ireland, Ryan who plays for Shelbourne, Gavin and Sean for Drogheda United.
