I think with a slightly implausible challenge I think a big POD is useful. So what if the Spanish Armada has better luck with the wind, and even after losing alot of ships to Drake; it ends up pulling off a successful invasion of Ireland.
http://en.wikipedia.org/wiki/Spanish_Armada_in_Ireland
The Spanish would do well enough on land that the Britishand their plantations are cast off the island. I don't know how it goes from there, but it would be interesting if the Irish would be used as replacement conquistadors on the fringes of the Spanish empire.
Sorry to nitpick, or change the subject in general, but it was the English, not the British, that ruled Ireland with the assistance of Anglo-Irish nobility.