Complex sentence: O'Brien's Tower is a round stone tower near the midpoint of the cliffs built by Sir Cornelius O'Brien, a descendant of Ireland's High King Brian Boru, to impress female visitors.
Split into simpler sentences: 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.