Just across the Bann river from the end of the Portstewart strand is Castlerock.
With accommodation and with a train stop on the main Belfast to Derry line. Unfortunately, there is no way to cross the river (although there has been talk of a future footbridge connecting the two) despite it seeming so close to the end of Portstewart. To get here you will have to drive via Coleraine.
Really helpful and nice people! Wouldn't go anywhere else to rent a student house!