Trilemma in the context of "Dilemma"

The Irish Sea border is an informal term for the trade border between Northern Ireland and Great Britain. It was specified by the Ireland/Northern Ireland Protocol of the Brexit withdrawal agreement (February 2020), was refined by the Joint Committee in December 2020, and came into effect on 1 January 2021 following the end of the Brexit transition period. As a result of the Agreement, Northern Ireland remains aligned to the European Single Market in a limited way for goods, whilst remaining part of the United Kingdom customs territory and the UK internal market. Its effect is that the need for customs checks on the Irish border has been avoided, and a hard border has not been re-established.

This Irish Sea border was the option taken by Prime Minister Johnson in October 2019 to break the impasse of the "Brexit Trilemma" (of three competing objectives: no hard border on the island; no Irish Sea border; and no British participation in the European Single Market and the European Union Customs Union: it is not possible to have all three.)

The Mundell–Fleming model, also known as the IS-LM-BoP model (or IS-LM-BP model), is an economic model first set forth (independently) by Robert Mundell and Marcus Fleming. The model is an extension of the IS–LM model. Whereas the traditional IS-LM model deals with economy under autarky (or a closed economy), the Mundell–Fleming model describes a small open economy.

The Mundell–Fleming model portrays the short-run relationship between an economy's nominal exchange rate, interest rate, and output (in contrast to the closed-economy IS-LM model, which focuses only on the relationship between the interest rate and output). The Mundell–Fleming model has been used to argue that an economy cannot simultaneously maintain a fixed exchange rate, free capital movement, and an independent monetary policy. An economy can only maintain two of the three at the same time. This principle is frequently called the "impossible trinity," "unholy trinity," "irreconcilable trinity," "inconsistent trinity," "policy trilemma," or the "Mundell–Fleming trilemma."