He is best known for his work with Greg Hemphill on the BBC Scotland sketch comedy series Chewin' the Fat (1999–2005) and the sitcom Still Game (2002–2007, 2016–2019).
Kiernan's career in television began when he started to write various sketches for Pulp Video (BBC Scotland 1995).
Between 1999 and 2005, he was a writer for Chewin' the Fat, starring as a regular character alongside Greg Hemphill, both playing various roles.
The spin-off series from Chewin' the Fat, Still Game, began broadcast in Scotland only, but later reached a wider audience throughout the United Kingdom.
[8] On 24 October 2013 Kiernan confirmed in the Daily Record that Kenny Boyle's tour of the original stage show had been one of the instigating factors for Still Game's return.
[9] The 21 shows at The Hydro ran from 19 September 2014 until 10 October 2014, played to 210,000 fans and made £6,000,000 in ticket sales.
On 13 July 2018, the BBC announced that Still Game would return for the ninth and final series later in 2019, after which the show will end.
The second stage show was to run for ten nights beginning 4 February 2017,[17] but in October 2016, a further five performances were added.
[21] In 2007 and 2008 Kiernan starred in Dear Green Place, a BBC One Scotland sitcom about park wardens.
In October 2011 Kiernan played Gordon Brown in The Hunt for Tony Blair, a one-off episode of The Comic Strip Presents...
In 1996 he starred alongside Robert Carlyle in Carla's Song and in 2002 he played the role of Black Joke Chief in the Martin Scorsese film Gangs of New York.
Kiernan lends his voice to the character of Banjo Barry in the animated film Sir Billi.
In 2020, Kieran played the role of Gavin in the American comedy movie Then Came You, alongside Craig Ferguson who had appeared with Kiernan in Still Game three years earlier.