The Republic of Ireland international told Villa's website: "I've been waiting for a while to get out on loan. Freddy Bouma has been in great form this season and this has meant I haven't had much of a look-in.
"I'm looking to get some games under my belt at Southampton and help them push up the Championship table.
"They are looking towards the Premier League and if I can help them in that aim, that would be great.
"The Saints are a big club with an impressive history and I'm thoroughly looking forward to my month here.
"George Burley is a well-respected manager and I really enjoyed my first training session this morning.
"We've got Plymouth on Saturday and I'd be delighted to get into the team for that match."