Since the 1998 Good Friday Agreement, Northern Ireland has experienced a striking growth in the level of support for centrist, non-aligned political parties with support for such parties more than doubling from the first NI Assembly elections in 1998 by the latest Assembly elections in 2022. There has simultaneously been a notable decline in strong unionist and nationalist identification and a growth in aligned voters voting for these centrist parties, both contributing to corresponding electoral gains for centre-ground parties. Brexit has intensified debate around these shifts, particularly in relation to a potential border poll and Irish unity. Yet the internal dynamics of centre-ground parties including how their activists navigate identity, ideology, and the constitutional question remain poorly understood. This thesis addresses that gap. The thesis examines the two parties constituting the contemporary NI centre-ground: the Alliance Party and the Green Party of Northern Ireland (GPNI). Drawing on forty-four semi-structured interviews with party activists, supplemented by archival research, media analysis, and polling data, it offers an intrinsic case study of what occupying the centre-ground means from the inside. The conceptual framework defines the centre-ground through two markers: civic orientation and constitutional neutrality on Northern Ireland's status while issue orientation is also critical for the GPNI. This framework identifies Alliance and the GPNI as the core instances of the centre-ground while preserving meaningful distinctions between them. Three central findings emerge. First, the centre-ground is populated by the less-aligned rather than non-aligned: activists who negotiate with, rather than transcend, inherited ethno-national identities. From Alliance, this manifests as residual communal attachments and a decline in British identification driven less by Brexit than by discomfort with unionism's inflection of Britishness. Second, Alliance and the GPNI approach constitutional neutrality differently. Alliance activists are heterogeneous on Irish unity, but broadly support organisational neutrality; GPNI activists, shaped by an eco-socialist ethos and operating as an issue-based party, are increasingly open to constitutional deliberation as a party responsibility, assessing options through the lens of environmental governance and post-growth economics. Third, both parties exhibit distinct conditionality cultures linked to commitments to inclusion and effective deliberation shaping the scope for future constitutional engagement.