Birch said: "I am looking forward to the prospect of playing on a regular basis and hopefully helping Southport to stay in the Conference." He arrived at Raydale Park from Carlisle in August 2003, after a three-year spell at Brunton Park in which he made 125 starts for the Cumbrians, and has played 104 times for Gretna.
Birch, who started his career with hometown club Stoke, helped Gretna win the Division Two and Division Three championships in consecutive seasons. Now he will aid Southport's battle against relegation. And he is set to make his debut in Tuesday night's derby with Morecambe.