The 26-year-old former Newcastle and Carlisle player, who has won five Republic of Ireland caps, made 26 appearances for the Robins this season and a total of 40 since joining the club on a free in July 2009 following his release from Hibernian.
Town boss Paul Hart told the club's official website: "Alan has had his contract terminated by mutual consent.
"He wasn't going to be part of what we did so we thought it would be fair to give him a chance to get ahead of the game in terms of finding a new club before the rush."
Source: PA
Source: PA