The Republic of Ireland Under-21 international will train with his new team-mates for the first time on Thursday.
Blues boss Gary Waddock told the club's official website: "He's a good young player who we've been keeping an eye on. As soon as we knew he was available for loan, we were interested.
"He's looking to play first-team football and he'll be a good addition to the group."
Kiernan progressed through the ranks at Vicarage Road, making his first-team debut in an FA Cup tie against Crystal Palace in January 2009 and has had loan spells at Kilmarnock, Yeovil and Bradford.
Source: PA
Source: PA