Carl, 21, comes to Home Park on a two-year deal after leaving Sky Bet League 1 Bradford City.
Like fellow summer signings Deane Smalley and Lee Cox, Carl has previously won promotion from League 2 - the season before last, when he was also starred in the Bantams’ incredible run to the final of the Capital One Cup.
A centre-back who can also play on the left, Carl made a total for 44 appearances for Bradford after joining them from Reading at the beginning of the 2012-13 season.
County Donegal-born Carl has represented Ireland at under-17, under-19 and under-21 levels, making his debut for the Under-21s in February last year and being included in his country’s squad for the Euro Under-21 Championship qualifiers in October.
Welcome to Home Park, Carl.