Brentford, of League One, have signed Kiernan from Premier League side Wigan Athletic until the new year
The 21-year-old centre back had been on loan at League Two side Burton Albion and now joins the Bees until January 2, 2013.
Speaking to the club's official website, Brentford manager Uwe Rosler said that he had been trying to secure Kiernan's signature for quite a while.
He said: "Rob is a player who I know, one who has been with us before. We had him here last summer and he has made big strides at Wigan since then.
"He has been on loan to Burton and now Roberto Martinez and everybody at Wigan think he is ready for the next step. We have been looking for somebody who has the qualities that Rob has, and that is why we have brought him in.
"I know what Rob is about and I have already seen him in training. I have got a good relationship with Roberto Martinez and we have been following Rob closely, not just now when we have been trying to get him but for a long time. We never really lost contact, I was always asking after Rob," added the 44-year-old.