"Here lies the remains of Catherine Moran… who closed her mortal career February 1st 1805" reads the gravestone in the church yard.

“‘Closing your mortal career’ – it’s a very local expression,” says our guide John. “You won’t find it anywhere else.” We’re on Scattery Island in the Shannon Estuary, West of Ireland, on a tour of the island’s 6th-century monastic site and I’m taking a few days to explore the newly signed 128-mile (207km) looped drive: the Shannon Estuary Way.

Now uninhabited, Scattery Island sits near the mouth of Ireland’s longest river, the River Shannon, where the wide tidal estuary flows out into the Atlantic Ocean. As well as churches and a round tower, there’s a deserted...