Over the period 2017–2019, the United Kingdom's House of Commons accrued more powers than usual in controlling the executive with respect to the latter's negotiations over the United Kingdom (UK) – European Union (EU) Withdrawal Agreement, an international treaty. The government (the executive), in its turn, lobbied to endow Stormont (the Northern Ireland Assembly – NIA), restored on 11 January 2020, with a decisive voice on the long-term application of relevant EU law made applicable by the protocol to this agreement in respect of Northern Ireland. This article seeks to present an original methodology based on Nicklas Luhmann's autopoietic systems theory of society and Foucaldian discourse analysis as helpful when accounting for these moves by exposing, as relational and contingent, interactions of the Members of Parliament in the House of Commons (MPs), as well as Members of the NIA (MLAs), with the central government in London. Westminster and Stormont, as well as the Supreme Court of the UK (UKSC), are presented here as ‘entangled’ collective agents of institutional change in the UK political system, involved in a combination of ‘nested’ Brexit political games.