The Megalithic Portal (megalithic.co.uk) gives a grid reference of SW 514 298 and says it is on the edge of the cliff, below the castle, on the south side of the island.
Original Chapel of St Nicholas situated on The Island St-Ives
Grid reference, SW 5196 4115
For further information see Historic Environment Record, https://www.heritagegateway.org.uk/Gateway/Results_Single.aspx?uid=MCO10227&resourceID=1020