Moher Tower is an old lookout tower at the southern end of the Cliffs of Moher, built as a watch against a Napoleonic invasion. Similar signal towers were put up all along Ireland's west coast, passing messages to each other by flag.
It's also the best way to see the cliffs without the crowds and the high parking charges at the main visitor centre. Park at Kilconnell for a small fee and walk the clifftop path north: it takes about two hours to reach the visitor centre, with the cliffs growing higher as you go. Or just stroll out to the tower and back.