The twenty-seventh, we marched by the famous Lough Leyn, out of which the ryver of Lowgen doth spring, and falleth into the sea beside Magne. The Logh is fulle of salmon, and hath in it eleven islands, in one of which (Innisfallen), there is an abbey in another a parish church, and in another (Rosse) a castel, out of which there came to us a fair lady the rejected wife of Lord Fitz-Maurice, daughter to the late McCartie-More (elder brother to the Erle). It is a circuit of twelve miles, having a faire plaine on one side, faire woodes and high mountaynes on the other side, thence we passed bv the entrie of Glanflesk, that "famous Spelunck," whereof the traytours make their chief fastnesse, and, finding neither people nor cattel there, we held on and camped that night in O'Kallaghan's countrie, by the river of Brode water which passeth by Youghal.Not surprised that the expedition cane around the eastern side of the lake in the end.
The rejected wife of Lord FitzMaurice is Katherine, daughter of the Earl's brother Teige (who most sources give as younger rather than older). Her estranged husband isn't James Fitzmaurice but Thomas Fitzmaurice, Baron Kerry. She was his second wife; she died of smallpox in 1582 and he married again.
I can't find any recent mention of the traitors' cave in Glenflesk, which is a shame as it sounds rather fun. It was being shown to tourists in 1846, anyway.