Complex sentence: Under James I, Catholics were barred from all public office after the gunpowder plot was discovered in 1605; the Gaelic Irish and Old English increasingly defined themselves as Catholic in opposition to the Protestant New English settlers.
Split into simpler sentences: Under James I, Catholics were barred from all public office after the gunpowder plot was discovered in 1605. In response, the descendants of the pre-conquest population increasingly defined themselves as Catholic (including the Old English) in opposition to the Protestant New English settlers.