O'Brien's Tower is a round stone tower at the approximate midpoint of the cliffs. It was built by Sir Cornelius O'Brien, a descendant of Ireland's High King Brian Boru, in order to impress female visitors. From atop that watchtower, visitors can view the Aran Islands and Galway Bay, the Maum Turk Mountains and the Twelve Pins to the north in Connemara, and Loop Head to the south.
The cliffs take their name from an old fort that once stood on Hag's Head, the most southern point of the Cliffs. The writer T.J. Westropp refers to it as Moher ui Ruis or Moher ui Ruidhin The fort still stood in 1780 and is mentioned in an account from John Lloyd's a Short Tour Of Clare (1780). The field where the fort stood is still called Moher a Thairbh. The present tower built near the site of the old Moher ui Ruidhin was built as a lookout tower during the Napoleonic wars