Questions about a united Ireland come up in every generation, and they returned with force after Brexit. Most debate is about whether unity is desirable. This explainer is about something narrower: what the law says about how it could happen.
The consent principle
The Good Friday Agreement of 1998 settled the basic rule. Northern Ireland remains part of the United Kingdom unless a majority of people voting in Northern Ireland choose otherwise in a referendum. If they do, and a majority in the Republic of Ireland agree in a separate vote, both governments are bound to give effect to that choice.
The UK side of this is in the Northern Ireland Act 1998. Section 1 and Schedule 1 say the Secretary of State may order a poll at any time, and shall order one if it appears likely to him or her that a majority of those voting would choose a united Ireland. Polls cannot be held more often than once every seven years.
The trigger is a judgement
The Act does not say how the Secretary of State should decide whether a majority is likely. In a judicial review brought by the victims' campaigner Raymond McCord, the courts in Belfast were asked to make the government set out a policy on this. They held that the minister must act honestly and rationally on the evidence, but declined to impose a formula. That leaves room for argument about which evidence counts: elections, opinion polls, census results, or something else.
The last border poll
Northern Ireland has had one referendum on the border, in March 1973. Almost 99% of those voting chose to remain in the UK, but nationalist parties called for a boycott and many Catholic voters stayed away, so the result says little about opinion at the time.
What has changed
The 2021 census was the first in which more residents were recorded as being from a Catholic background (45.7%) than a Protestant one (43.5%). Religious background is not the same as constitutional preference, and polls have consistently shown a sizeable group who identify as neither unionist nor nationalist. Supporters of unity point to the trend; opponents point out that polls have not shown a stable majority for change.
Why it matters here
Arguments about the constitutional future and arguments about the legacy of the conflict are linked. Unionists and nationalists read the record of collusion, including in the Finucane case, very differently, and both sides draw on it in debates about trust and reconciliation.