This section is from the book "Ancient Legends, Mystic Charms, And Superstitions Of Ireland", by Jane Francesca Wilde. Also available from Amazon: Ancient Legends, Mystic Charms, And Superstitions Of Ireland.
Lough Foyle means the borrowed lake, for in old times there were two weird sisters dwelling beyond the Shannon, who were skilled in necromancy. And the elder sister said to the younger-
" Give me the loan of your silver lake, for I have none ; and I promise to restore it to you next Monday."
So the younger, being good-natured, rolled up the lake in a sheet and despatched it over hills and dales to her sister. But when the time came for return, the elder sister, being deceitful and cunning, made answer to the messenger sent for it-
" Truly, I said Monday, but I meant the Day of Judgment. So I shall keep the lake till then."
And the lake therefore remains in her country to this day, while the great hollow whence it was taken can still be seen in Connaught, bare and barren, waiting for the waters that never will return.