And after 1776, the Scots Irish were willing to hang around for long periods unpaid, making them the ideal Revolutionary soldiers!
Indeed, but for many who joined Washington's Continential Line, pay was a major motivating factor. Charles Patrick Neimeyer's America Goes to War: A Social History of the Continental Army is an excellent source discussing how Washington needed to create a "professional" fighting force that could stand up against the British regulars than the militia, which did not have the discipline to do so.

While the Line certainly kept the Revolution going, as Washington's Army became the cause itself (destroy the army, and you destroy the movement), some (but not all) who joined did so need of a paycheck, especially working-class laborers, seamen, etc. and some were even forced to serve, such as Loyalists in Patriot strongholds.

Don Higginbothan's George Washington and the American Military Tradition is also highly recommended. Higginbotham's discussion of Washington's desire for a commission in the British Army is most interesting, and it certainly makes you wonder "what if...".

T.