This article aims to broaden the theoretical foundations of the two-level games approach to understanding international negotiations by considering the conditions under which public opinion can act as a domestic constraint on the ability of international negotiators to reach agreement. In determining the role that public opinion plays, three factors are of central importance: (1) the preferences of the public relative to those of decision makers and other domestic constituents; (2) the intensity of the issue under negotiation; and (3) the power of the public to ratify a potential agreement. Evidence from the last decade of Anglo-Irish negotiations over the future and status of Northern Ireland shows that public opinion acts as a constraint on negotiators when the public has the power to directly ratify an international agreement. When the public's power to ratify an agreement is indirect, the intensity of the issue under negotiation will play a critical role in determining whether public preferences serve as a constraint on decision makers.