Pronunciation question.

I will be reading my grandfather's favorite poem, Yeats' "The Lake Isle of Innisfree," at his interment on Monday. I am American. I'd like to know the correct pronunciation of the place name "Innisfree." All of the readings that I have heard pronounce it with an "s" sound, although I suspect that it might be correct to use an "ish" sound. Any advice?

EDIT: "Inn-ish-free" it is! Thanks for all of the help, friends. We'll raise the parting glass when he's resting in peace.