We have visited Ballycastle in County Antrim on a few occasions. As far as the North Coast goes if I had a choice between visiting the Giant's Causeway or stopping in Ballycastle I would always choose the latter.
A highlight of any trip to Ballycastle is getting out on the water. Book well in advance for the ferry to Rathlin Island and allow at least 4 hours to see and walk the island.
The facilities on Rathlin are sparse and sometimes it can be hard to even get a bite to eat. If they are open you can get good food in McCuaigs bar or The Manor House Hotel. However the best grub can be had from a small food truck called The Hungry Seal. The fish and chips are delicious and even better enjoyed sitting on the nearby beach wall. All of the amenities are dotted around the harbour where the ferry lands.
Further out from the harbour there are three lighthouses, a bird sanctuary, and plenty of really nice walks to enjoy on the island. Check out the photos we took on Rathlin the last time we visited.