I will add that the association of Ulster Scots Protestants with loyalism dates only from the late 19th century, when Irish nationalism was taken over by Catholics. In the 18th century, many Irish Protestants felt persecuted by the British Crown, since their Presbyterian faith was not officially tolerated outside of Scotland, one reason why so many emigrated to America and later supported the rebellion. Not only in America, but also in Ireland, where the leader of the 1798 Irish uprising, Wolfe Tone, was a Protestant.
Of course, the Ulster Scots phenomenon was in origin a deliberate policy of subjugating Irish Catholics by planting Protestant settlers from Lowland Scotland and the Borders. Ulster Scots loyalists today naturally place a lot of emphasis on this early history, and less on the 18th century.
Thanks Iconodule for anticipating this.
Also, there are two kinds of Irish Protestants: the true Ulster Scots, who inhabit the North and are Presbyterians, and the Anglo-Irish, who are more evenly distributed throughout the island and are mostly a type of Episcopalian/Anglican.