The island itself comprises 185 acres of grassland with a freshwater lake and several springwater wells. It slopes gently from sea level on the east to a height of 110 feet over two kilometers to the west where the watchtower is situated. The east of the island contains the ruins of two cottages, a disused graveyard and the site of a cross and oratory, as well as several walled fields.
According to tradition Mutton Island gets its name from the sheep that landed there off a wrecked Spanish Armada Ship called the Zuniga, which floundered on its coast in 1588. Some of the people from the ship are buried in the disused graveyard and a brass cannon from the ship was still on the island up to thirty years ago. Two promontory forts on the island attest to it's prehistoric inhabitation when it was at the outer edge of the known world.
By 548 AD the first recorded building was erected there by St. Senan who built a church and founded a monastic settlement there while on his way from the Aran Islands to the Shannon Estuary.