Mrs Clinton addressed the Northern Ireland Assembly, telling a hushed, packed chamber that Republican dissidents were looking to seize any opportunity to destabilise the coalition government.
"Now they are watching this assembly for signs of uncertainty or internal disagreement," warned Mrs Clinton. "They want to derail your confidence. And though they are small in number, their thuggish tactics and destructive ambitions threaten the security of every family in Northern Ireland. Moving ahead together with the process will leave them stranded on the wrong side of history."
Almost all of the 108 members of the assembly applauded, but a few Democratic Unionist backbenchers folded their arms instead, and two senior figures, William McCrea and Gregory Campbell, left the chamber during the ovation.
Democratic Unionist officials said the walkout reflected Protestant irritation at being told what to do by 'outsiders', a point they said they had made earlier in private to Mrs Clinton.
Mrs Clinton conceded this sensitivity in her speech, ad libbing: "We know what it means to be supportive. And we also know what it means to meddle." She said that the US sought to do the former, not the latter.