The 25-year-old Republic of Ireland centre-half, who joined Reading from Cork City in January, has been training with Saints this week and is in the squad for tonight's friendly at Millwall.
Saints boss George Burley has been on the look-out for defensive reinforcements following the departures of Gareth Bale and Chris Baird, while Claus Lundekvam is injured and Darren Powell is suspended for the first three games of the season.
"Alan is a great lad, a fantastic character and an experienced professional who made his name with Cork," said Burley.
"He did very well for Reading last year in their reserve team and is coming here looking for first-team games."
Royals boss Steve Coppell added: "This is a great opportunity for Alan to get regular first-team football at a club with a rich history and serious designs on promotion."