The estate, on Anglesey, also includes a six-bedroom house, the ruins of a clay works and a private beach, was put up for sale for £895,000. The uninhabited island, Ynys Badrig, or Patrick's Parish, measures two acres and lies just off the remainder of the estate. It is thought St Patrick, the patron saint of Ireland was shipwrecked on the island. In AD440, Patrick was returning from a visit to Saint Columba on Iona when his boat was wrecked on the island, also known as Middle Mouse.
He swam the treacherous mile to Anglesey and lived in a cave.
Legend says St Patrick built the church of St Llanbadrig in thanksgiving for his deliverance. From the sunny graveyard, you can look out to the north in clear weather and see the blue crests of the hills of the Isle of Man.
Andy Woods of Beresford Adams, which is selling the estate, said: "Of course we can't be sure about the links with St Patrick and we would not be putting any such thing in the sales material because that would be breaking the law.
"The house itself needs a bit of attention but the area is outstanding. It's in National Trust land and so you don't have any neighbours, except for seals and birdlife, for miles. It is real one-off.
"There can't be many places where you get a beautiful house and stunning scenery, together with your own island with links to one of the world's most loved saints."
St Patrick was said to have driven the snakes out of Ireland, and his legend is celebrated across the globe on 17 March each year.
The house has been the centre of a working farm until recently and was put on the market after the owner died.
It has already attracted about 20 potential buyers with about 45 others expressing an interest.
There is some potential to turn the disused clay works into a tourist attraction but the agents doubt whether planning permission would be granted given the precious ecology of the area. "It is more suitable for a wealthy family who want to get away from it all.
"It would naturally suit outdoors types and there is great diving off the coast," said Mr Woods.