The appeal of constitutional referendums as legitimating tools revolves around their participatory promise: by triggering large scale participation and possibly also deliberation, they are said to inculcate national ownership. Recent work has also argued in favour of referendums’ potential for deadlock breaking and even peacemaking in divided and post-conflict societies. This paper investigates a common occurrence in such instances: referendum boycotts. One or several of the key players in the polity may call for a boycott in order to block or delegitimise the constitutional reform process. The international community itself may see a boycott as evidence of the process’s lack of democratic credentials. This paper draws on comparative practice to investigate the questions raised by boycotted referendums, options to avoid or manage boycotts, and how such results are to be interpreted. The backdrop to this is the possible vote on Irish unification and the risk of a boycott therein.