I left Knock airport provisioned with two bottles of the hard stuff - holy water from the Shrine Shop - and a packet of Lucky Sawdust for the Traveller to cover all eventualities.

The terminal was built to enable pilgrims to visit the shrine marking a 19th-century sighting of the Virgin Mary, but I was on a pilgrimage of another kind. I was heading for the wild west of Ireland to visit a pirate queen's haunt 60 miles out to sea. Clare Island was the stronghold of feisty Grace O'Malley, who controlled these coastal waters 400 years ago.

At tiny Roonagh Quay, 18 miles south of Westport, two ferries, the Island Princess and the Pirate Queen, were moored alongside. We were handed flyers from rival Portakabins above the pier. "The ferry service was originally run by the O'Gradys, but then the O'Malleys got in on the act," I was told by a young O'Grady on board the Pirate Queen.

After 15 minutes onboard, a humpy long-necked island necklaced with golden sand surged into view. The harbour was guarded by O'Malley's sturdy castle, a square refuge crowned by two chimneys, its slit windows commanding strategic views of Clew Bay.

As we found to our cost, the weather changed every few minutes. Heavy bursts of rain were followed by apocalyptic sunbursts accompanied by a succession of rainbows that leapfrogged across the islands.

We approached the remains of a Cistercian abbey, its grilled gateway fastened with a bicycle lock. A churchyard overflowed with the graves of generations of O'Malleys. I knocked on the door of a tiny cottage and was given the key by an old man. The walls of the abbey were decorated with medieval imagery. On one wall was the stone tracery of a tomb and a plaque carved with the motto terra marique potens - "powerful by land and sea". This is the O'Malley coat of arms, and the tomb is reputed to be that of the pirate queen. As I photographed the stone carving, a gust of wind slammed the abbey door shut, plunging us into darkness.

I returned the key to its owner. "Would you be an O'Malley now?" he wondered. I wasn't, but O'Malleys from all over the world come to visit their ancestor's island. "Are there any O'Malley pirates left?" I asked the guardian of the key. "Only the one with the ferry," he replied, showing several teeth.

Ryanair (0870 1569569, www.ryanair.com) flies to Knock from London Stansted from £123. From the nearest railway station to Knock, Claremorris, (Irish railways, Iarnród Éireann, tel: 00353 1836 6222, www.irishrail.ie), operates services to Westport, standard return £10.50. The ferry from Roonagh Quay to Clare Island costs £20 return. For information, contact Chris O'Grady on 098-26 307 (Bay View Hotel) www.anu.net/clareisland/pirate.htm

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