After opening Lough Derg, we passed by Holy Island, with its ruins and round tower looming in the distance. The island contains about twenty acres, and so valuable are the feed derivable from the host of penitents who repair to do their stations on the Holy Isle, that the ferry, between it and the main, is rented for a considerable annual sum.
JK [Sir James Emerson Tennent Bart]Letters to the North, from a Traveller in the South Hodgson, Belfast; Milliken and Son, Dublin 1837
Irish waterways history 
Have been to Holy Island, it is a beautiful place.