800 years is a long time. It’s so long ago that:
- There were no Protestants. There were some faiths that evolved into Protestantism through more or less convoluted paths, but the Protestant Reformation was a good 2–400 years later.
- The Roman Catholic Church had only been invented a couple of centuries previously following the Great Schism, and wasn’t the power it became.
- Ireland wasn’t really Catholic even by the standards of the time (see above).
- Ireland also wasn’t really Ireland, but a variable number of warring kingdoms with a more but mostly less effective referee (High King).
- There was no Britain. There was barely an England, and it was French.
Now that I’ve cleared that up for you, I’m sure you can see your way to the answer.
Okay, the English and some not-quite-onboard Welsh – who were all actually Frenchmen descended from Vikings – went to not-Ireland to help one of the warring kings of not-Ireland against all the other kings of not-Not-Ireland and the referee-king. Following the not-invasion and ensuing not-conquest, the French Viking not-English-and-not-Welsh brought not-quite-Catholicism to some parts of not-Ireland. Not-Ireland had signed up about 100 years previously to the not-quite Catholic Church – having been a lot closer doctrinally to the Orthodox Churches from which the not-quite Catholic Church had recently schismed – but the not-Irish had signed up to the even-more-not-quite-Catholic Church on several occasions since Patrick popped his pampooties and had not followed through, so the not-quite-Catholic Church was keen for a little more cooperation and enforcement on the matter.
Through centuries fraught with attempted takeovers by the Germans and Spanish and actual takeovers by the Welsh, Scots, Dutch and the Germans (more successfully this time), French-England-and-Wales became Great Britain, then not-proper-Catholic, then Protestant, then proper-Catholic-as-understood-at-the-time, then Protestant again, then maybe-Catholic, then ultra-fundamentalist-Protestant, then who knows under Charles II, then Catholic, then Protestant, where it finally stuck. During this period, not-Ireland became Ireland under British rule, and the natives progressively more Catholic. This happened largely without the nation-wide convulsions seen over the water until the very end, when the last British Protestant-Catholic war was fought in Ireland by a Pope-backed Protestant against a Franco-Scottish Catholic. Such was the confusion engendered by this bizarre confluence that they forgot to take it with them when they went back to Britain – or rather, to continental Europe, where both William and James (titular kings of Britain and Ireland) spent much of the rest of their lives.
Now: William, King Billy, was more concerned with getting his big white horse over to the continent to continue his wars there, than he was about his English throne or Ireland, and signed a surprisingly modern, non-punitive treaty to end the war in Ireland. Unfortunately, Parliament, frankly knackered from a century and a half of turmoil which had seen them variously kicked out of their jobs, hung drawn and quartered, burned as heretics, or on the run in foreign parts, decided they simply were Not Having That. Hence, the Penal Laws.
Lest ye be thinking the mud of confusion is finally beginning to sink to the bottom of the flowing stream of this narrative, fear not: the Penal Laws didn’t just affect the Irish (they applied in Scotland, England and Wales too) or even just Catholics (anyone who wasn’t a member of the Anglican faith was subject too – Presbyterians, Quakers, Jews…). Partly as a result of this, many of the Irish rebel leaders and agitators for home rule and/or Irish independence from then onwards were in fact Protestant, including Wolfe Tone, Robert Emmet, William Smith O’Brien, Charles Stuart Parnell, Erskine Childers, and Douglas Hyde.
It was really only after partition that the conflict coalesced into a Protestant vs. Catholic division, and, even then, only in Northern Ireland. The religious terms gloss the two traditions, and have come to be applied, retrospectively and inaccurately, to the demented carousel that is Irish history.