Mirissa is a small beach town and a fishing port located on the Southern tip of Sri Lanka. The warm Indian Ocean is the only thing between you and the next large land mass, the continent of Antarctica or Africa.
Colombo, the capital city of Sri Lanka and your most likely entry point onto the Island unless you are swimming is about 150 km North West. This translated into roughly a 3-4 hour trip by road depending on traffic and how insane your driver is.
Mirissa Beach is sandwiched between the towns of Welligama and Matara and it is about 35 KM or 1 hour South of the fort city of Galle by bus.