Date of birth
Republic of Ireland
Kiernan joined from Scottish Premier League side Rangers having previously been on loan at Roots Hall in 2013.
The 26-year-old played 15 games at the club before being recalled due to an injury at his parent club Wigan Athletic.
But the 6ft 1in defender opted to once again join Blues from Rangers in the 2017 summer transfer window.
Hertfordshire-born Kiernan, who has represented Republic of Ireland at youth levels, began his career at Watford but didn’t make many appearances in the first team, instead spending time on loan at Kilmarnock, Yeovil Town, Bradford City and Wycombe Wanderers.
He moved to Premier League side at the time Wigan Athletic in 2011.
Further loan spells followed, including at Blues, before a move north of the border to Rangers for a fee in the region of £200,000.
At Rangers Kiernan played 76 games in two years and he will wear the number 15 shirt this season.