Manager Derek McInnes moved for the experienced Irishman after losing Dave Mackay and Danny Grainger to injury.
The Saints' defensive options for the weekend are further limited by the suspension of Steven Anderson.
"The injury and suspension situation has highlighted the need for someone but this is no short-term signing," McInnes told the club website.
"Alan will bring something to the squad for the entire season - that's why we have given him a year's contract.
"He knows the league and will bring a experience, versatility and ability to the squad."