Take either of the ferries from Ballycastle to Rathlin to begin the journey (bikes cost extra).
Take a left at the harbour and follow the road a short distance to St Thomas church and turn right onto a steep climb. Follow this road on up to the left, passing by the immigration memorial stone which can be seen at the side of the road. Follow the road up to the West Lighthouse and bird sanctuary, taking in the spectacular views of the North Antrim coast to the south, Donegal to the west and Islay to the north.
Return along the same road and take the road to the left of St Marys chapel, this will take you to the East Lighthouse (Altacarry), with fantastic views of Islay, Gigha, the Mull of Kintyre, Sanda, Ailsa Craig, mainland Scotland (on a clear day) then down towards the Maidens lighthouse and east Antrim coastline with Torr Head, Murlough Bay and Fairhead easily recognisable. There is also the remains of Bruce's Castle here and Bruce's Cave is just to the south of the lighthouse although not accessible on foot.
Back along the same road and take a left which will eventually take you back down to Church Bay. Follow the road at the rear of the bar towards the south of the island and Rue Point lighthouse. A couple of miles along this road there is a waymarked nature trail on the right at Roonivoolin. This is all off road and takes you along the cliff edge (not too close!) and then back onto the main road a short distance from Rue Point. Follow the road down to the shore here to see the ruins of the "smugglers" houses and the lighthouse itself. There's usually a lot of seals to be seen in the natural harbour here.
Retrace your steps back towards the harbour, taking the road to the left which will take you down past Mill Bay (again lots of seals here) and the ruins of the old kelp store and finally back to the harbour where you can stop off in a number of places for refreshments, before getting back onto the boat.
Total distance is around 20 miles and it's pretty much all on roads except for the trail at Roonivoolin.