Though I do not have Plutarch's original Greek text to hand, I can only assume that the word he used was ανευθυνος [a)neu/qunos], 'not accountable, irresponsible'. (OK, it might've been ανυπευθυνος [a)nupeu/qunos].) From ευθυνω [eu)qu_/nw] 'to guide straight or direct; govern'. It occurs in Aristotle, Plato, and Thucydides. Googling on "irresponsible power" or "irresponsible sovereignty" yields other citations than Dryden's.
"Irresponsible power is inconsistent with liberty, and must corrupt those who exercise it." [John Calhoun, US lawyer and politician (1782-1850)]
One of the meanings of irresponsible in current dictionaries is still one of not being held accountable by a higher authority. (Nor is this meaning censured with an archaic or the like.) I don't see that the language has been impoverished, just some of its speakers no longer make the distinction.
