Fenagh sits on the site of a monastery founded by Saint Caillin in the 6th century. What you find now is the shell of that long history: the remains of St. Caillin's Church, the ruins of Fenagh Abbey (medieval), and what may be a round tower, though the landscape has reclaimed much. The Book of Fenagh—a 15th-century manuscript recording the saints and history of the monastery—remains an important medieval source.
The village is very small, honest in its scarcity. The significance here is archaeological and spiritual, not commercial. Come to walk among the ruins, to sense the layers of Irish monastic life, to understand how these places were centres of learning and prayer for a thousand years.
The reason to come back. The things every local will eventually tell you about, usually after the second pint.
Wear waterproofs. Bring a sandwich. Tell someone where you're going if it's the mountain.