Standing guard over Inishmore, Dun Aengus, 7km west of Kilronan, has three massive drystone walls that run right up to sheer drops to the ocean below. Originally built around 1100 BC, it was refortified around 700 AD and is protected by remarkable chevaux de frise, fearsome and densely packed defensive limestone spikes. Displays at its small visitor centre provide context and a 900m walkway wanders uphill to the fort itself.

Powerful swells pound the 87m-high cliff face. A complete lack of railings or other modern additions that would spoil this incredible site means that you can not only go right up to the cliff's edge, but also potentially fall to your doom below – take care.