The only island of size is the L-shaped Rathlin Island , off Ballycastle , 11 km ( 6.8 mi ) in total length by 2 km ( 1.2 mi ) maximum breadth , 7 km ( 4.3 mi ) from the coast , and of similar basaltic and limestone formation to that of the mainland .
The only large island of County Antrim is L-shaped Rathlin Island , off Ballycastle , 11 km ( 6.8 mi ) in total length by 2 km ( 1.2 mi ) maximum breadth , 7 km ( 4.3 mi ) from the coast .