Not sure how accurate this encyclopedia is but seems pretty well sourced
http://www.newworldencyclopedia.org/entry/Battle_of_the_Boyne
taken from that source above:
The battle of the Boyne was the decisive encounter in a war that was primarily about James's attempt to regain the thrones of England and Scotland, but is widely remembered as a decisive moment in the struggle between Protestant and Catholic factions in Ireland. However, recent analyses have played down the religious aspect of the conflict. In fact, both armies were religiously mixed, and William of Orange's own elite force—the Dutch Blue Guards—had the papal banner with them on the day, many of the Guardsmen being Dutch Catholics. They were part of the League of Augsburg, a cross-Christian alliance designed to stop a French conquest of Europe, supported by the Vatican.