Kiernan started his career in the youth team at Fulham, but left as a youngster to join Crystal Palace. He stayed with the club until he was 16, but was released at the end of the season in 2009. He then joined the AFC Wimbledon Senior Academy at South Thames College, where he got picked for the England Colleges team.
AFC Wimbledon
The midfielder made his league debut for "The Dons" in a Conference game against Crawley Town in a 3–1 defeat on 18 March 2011, coming on as a substitute for Luke Moore. Kiernan only featured in one more game in the 2010–11 season, a 0–0 draw with Forest Green Rovers, which was his first League start for the club, securing a place in the Conference play-offs. Following AFC Wimbledon's promotion to the Football league Kiernan made his football league debut on 24 September 2011 when he came on as a substitute for Luke Moore in a 2–1 win over Bradford City in League Two. On 27 January 2012, Kiernan signed for Braintree Town on a one-month loan deal, making four appearances in total. Kiernan scored his first goal in professional football in the First Round of the League Cup against Stevenage on 14 August 2012. It would only prove to be a consolation, however, as AFC Wimbledon went on to be defeated 3–1. Having broken into the first team under manager Terry Brown at the start of the 2012 season, Kiernan left the club by mutual consent on 8 April 2013 under new manager Neal Ardley.
Following his departure from AFC Wimbledon, Kiernan joined his local club Conference South side Bromley in July 2013. After impressing in pre-season, he scored on his debut, netting the first goal in a 2–0 away victory against Hayes and Yeading United on 17 August 2013. A week later, he scored Bromley's third goal in a 5–1 win over local rivals Tonbridge Angels. His next goal came on 7 December, in a 3–0 home win over Farnborough. He assisted 14 goals in the league during the 2013/14 season.
Ebbsfleet United
On 5 June 2015, Kiernan's contract with Ebbsfleet expired and he chose to pursue other opportunities for the coming season. His league campaign had been disrupted by injury but starred in the run to the quarter finals of the F A trophy, scoring against Welling and in beating Forest Green. . Following his departure from the club, he had brief spells with Lingfield and Hayes & Yeading.
On 29 September 2015, it was announced Kiernan would return to Bromley on a non-contract basis. He made his return debut on the same day against Dover Athletic in the Kent Senior Cup.and played against Boreham Wood in the league. Brendan decided to signed for Hampton and Richmond FC of the Ryman League in October 2015, going on to make 31 appearances, scoring 12 goals and 15 assists, helping Hampton to win the league and promotion to the conference South.
Harrogate Town
After a season at Welling United, it was announced in May 2019, that Kiernan had joined Harrogate Town.
