As to Ireland, that is still a distinct kingdom, though a dependent subordinate kingdom. It was only entitled the dominion or lordship of Ireland, and the king’s style was no other than dominus Hiberniæ, lord of Ireland, till the thirty-third year of king Henry the Eighth, when he assumed the title of king.... But, as Scotland and England are now one and the same kingdom, and yet differ in their municipal laws, so England and Ireland are, on the other hand, distinct kingdoms, and yet in general agree in their laws. The inhabitants of Ireland are, for the most part, descended from the English, who planted it as a kind of colony, after the conquest of it by king Henry the Second; and the laws of England were then received and sworn to by the Irish nation assembled at the council of Lismore. And as Ireland, thus conquered, planted, and governed, still continues in a state of dependence, it must necessarily conform to, and be obliged by, such laws as the superior state thinks proper to prescribe.Ireland won legislative independence from Britain in 1782-3, under "Grattan's Parliament", but this ended with the union of 1801. The Act of Union, passed by the British parliament in 1800, is still on the statute book. It provided that
the said kingdoms of Great Britain and Ireland shall, upon the first day of January which shall be in the year of our Lord one thousand eight hundred and one, and for ever after, be united into one kingdom, by the name of the United Kingdom of Great Britain and Ireland.This statutory provision may be considered the root of title for British sovereignty in Northern Ireland. British sovereignty was reasserted by section 75 of the Government of Ireland Act 1920. It was this Act which partitioned the island of Ireland and established the entity of Northern Ireland, as of 3 May 1921. The status quo was reaffirmed more recently by section 1 of the Northern Ireland Act 1998:
1.— Status of Northern Ireland.Note that section 1(1) is purely declaratory - it affirms the existing situation rather than effecting any change in it.
(1) It is hereby declared that Northern Ireland in its entirety remains part of the United Kingdom and shall not cease to be so without the consent of a majority of the people of Northern Ireland voting in a poll held for the purposes of this section....
(2) But if the wish expressed by a majority in such a poll is that Northern Ireland should cease to be part of the United Kingdom and form part of a united Ireland, the Secretary of State shall lay before Parliament such proposals to give effect to that wish as may be agreed between Her Majesty's Government in the United Kingdom and the Government of Ireland.
The foregoing provisions were inserted into the 1998 Act under the terms of the 1998 Belfast Agreement, also known as the Good Friday Agreement. Section 1(1) enshrines what has become known in nationalist circles as the "unionist veto". Perhaps surprisingly, the principle that a united Ireland would not be imposed on the unionist population against its will postdates the foundation of Northern Ireland by some years. The scheme of the original Government of Ireland Act 1920 had been to create northern and southern jurisdictions in Ireland with a view to their eventual reunification. Only in the Ireland Act 1949 did the British authorities give unionists a veto over a united Ireland - indeed, Neville Chamberlain had unilaterally offered the de Valera government a united Ireland in 1940 as the price for Irish entry into World War II (de Valera refused, believing that the British government would be unable to deliver on the offer). The 1949 Act vested the veto power in the old Stormont Parliament. When Stormont was abolished, it was transferred to the Northern Ireland electorate as a whole by the Northern Ireland Constitution Act 1973.
The 1998 Act preserves the power of the UK Parliament, but allows the Northern Ireland Assembly to modify UK statutes insofar as they apply in Northern Ireland (s.5(6)).
The 1998 Act was described by Lord Bingham in Robinson v Secretary of State for Northern Ireland 2002 WL 1446153 as being "in effect a constitution". Its predecessor, the 1920 Act, had likewise been revered by unionists as the constitution of their state, even though they had originally opposed its passage.
The international perspective
What is the position of Northern Ireland in international law? The principle of national self-determination is recognised by the UN Charter (Chapter 1, Article 1, part 2), the International Covenant on Civil and Political Rights (Article 1) and the International Covenant on Economic, Social and Cultural Rights (Article 1). The problem, of course, is that no-one can agree on precisely how this principle applies to NI.
A more directly relevant international treaty is the Belfast Agreement, which took the overall form of an agreement between the British and Irish governments. As noted, the provisions of section 1 of the 1998 Act were enacted as a result of the Belfast Agreement, and the Agreement provided further that the British and Irish governments would recognise the legitimacy of whatever choice might be freely exercised by a majority of the people of Northern Ireland with regard to its status (Art. 1(i)). It also gave formal recognition to the unionist veto (Art. 1(ii),(iii)). Similar provisions were included in the 1985 Anglo-Irish Agreement. It can therefore be said that, as a matter of international law, British sovereignty over Northern Ireland is not unqualified, and has not been for some years.
From the Irish side, Articles 2 and 3 of the Constitution of Ireland currently provide as follows:
Article 2As is well known, the original 1937 version of the Constitution expressly asserted Irish sovereignty over "the whole of the island of Ireland" (albeit the legal effects of this assertion were slight, as the hardline republican politician Kevin Boland found when he tried to enforce Articles 2 and 3 in the Irish courts in Boland v An Taoiseach  IR 338). This was the subject of some resentment among unionists, who regarded it (with considerable justification) as an illegal extraterritorial claim. The amendments made following the Belfast Agreement appear to have replaced the earlier blunt assertion of sovereignty with a more diplomatic and less explicit affirmation of the Irishness of the North.
It is the entitlement and birthright of every person born in the island of Ireland, which includes its islands and seas, to be part of the Irish Nation. That is also the entitlement of all persons otherwise qualified in accordance with law to be citizens of Ireland. Furthermore, the Irish nation cherishes its special affinity with people of Irish ancestry living abroad who share its cultural identity and heritage.
1. It is the firm will of the Irish Nation, in harmony and friendship, to unite all the people who share the territory of the island of Ireland, in all the diversity of their identities and traditions, recognising that a united Ireland shall be brought about only by peaceful means with the consent of a majority of the people, democratically expressed, in both jurisdictions in the island. Until then, the laws enacted by the Parliament established by this Constitution shall have the like area and extent of application as the laws enacted by the Parliament that existed immediately before the coming into operation of this Constitution.
2. Institutions with executive powers and functions that are shared between those jurisdictions may be established by their respective responsible authorities for stated purposes and may exercise powers and functions in respect of all or any part of the island.