Political cleavages are often understood as deriving from either deep-rooted social divisions or institutional incentives. Contemporary Northern Ireland provides a test of the mutability of apparently entrenched cleavages to institutional change. Research undertaken before the ceasefire in the 1990s found noticeable asymmetries in the patterns of cleavage within the unionist and nationalist blocs. Within the unionist bloc, economic ‘left–right’ issues formed the main ideological division between the two major unionist parties. This contrasted with an ethno-national source of ideological division between the two nationalist parties. However, the emergence of a consociational form of government structure since then has demonstrated the ability of institutional incentives to reform some aspects of party competition swiftly. As evidence of this, we show that between 1989 and 2004 there was little change in the sources of support for Sinn Féin relative to the SDLP, but the influence of left–right ideology within the unionist bloc was negated as the influence of ethno-nationalism dramatically increased.