The 24-year-old is set to become new Rangers boss Mark Warburton’s first summer signing after a fee of around £200k was agreed between the clubs.
Warburton worked with Kiernan during their shared spell at Watford’s academy and he then signed Kiernan on loan for Brentford whilst working as the club’s sporting director, impressing in his eight appearances with the Bees during the 2012-13 season.
Kiernan, a former Republic of Ireland international, was nurtured further by current Everton boss Roberto Martinez whilst at Wigan, and he was tipped for a big future by the Spaniard.
But the centre-back, who can also play in central midfield, struggled to break into Wigan’s first team under Uwe Rosler and then Malky Mackay during a troubled last season in which the club’s chairman Dave Whelan left and they were relegated to League One.
Kiernan made 12 appearances on loan at Birmingham after joining the Midlands club for the rest of last season in February, and he is understood to be keen to move on to a new challenge.