The centre-back has played twice for the club following his initial one-month youth loan move from Watford in mid-February.
Manager Gary Waddock told the club's official website: "I'm really pleased to have Rob on board until the end of the season.
"We've seen his qualities during the month he's been at the club and he's a good player to have around the group."
Kiernan is away on international duty with Republic of Ireland Under-21s and is therefore unavailable for selection for Saturday's trip to Morecambe.
The deal includes a 24-hour recall clause.
Source: PA
Source: PA