The Boathouse Visitor Centre sits at the south end of the harbour at Church Bay, a minute from the ferry, and has a small historical exhibition on the island. It is a good place to start, because Rathlin's history is long and violent: a monastery raided by Vikings, Robert the Bruce in hiding, massacres in 1575 and 1642, and the cruiser HMS Drake, torpedoed in 1917, which sank in Church Bay itself.
It is free and opens daily in season, roughly April to September. Close by are the ruined Kelp House of around 1750 and St Thomas' Church of 1812, whose graveyard holds war graves of sailors from HMS Viknor and HMS Racoon.