The principle of consent is the cornerstone of the Northern Ireland peace process. The major breakthrough of the Good Friday Agreement was to get all parties to agree to this principle as the basis for further negotiation.
Basically this enshrines the right of the people of Northern Ireland to self determination: there will be no change in the constitutional status of the province without the consent of the majority of its inhabitants. That's strightforward democracy, you might say, why did it take them so long to come around to it? Well, it's a little more complicated than that...
First, you have to consider the legacy of imperialism: it may seem self-evident to us that the will of the majority should prevail, but this tenet wasn't exactly the cornerstone of the British Empire. By the end of the second world war, the imperial project was more or less discredited, but you have to allow for the fact that for the first half of the twentieth century, Britain maintained rule in many territories against the will of the indigenous majorities. This was not the case in Northern Ireland, of course, where the majority supported British rule, but it's important to remember that imperialism, while it lasted, was not compatible with the principle of consent.
Although the majority within Northern Ireland have always wanted to
remain a part of the United Kingdom, the minority wishing to join
in a united Ireland is significant, varying from about 33% to 40%
over the past few decades. What's more, the Catholic population
has been seen to be growing faster than the Protestant population, so Unionists have always feared that the day was not too far off when they would be in the minority. Extreme Unionists faced with this prospect would adopt the mentality of South African apartheid and ignore the wishes of the majority, invoking variously the divine right of the sovereign to rule Ulster, the superiority of Protestant people and culture, etc., etc. However, mainstream Unionists pride themselves on being reasonable people, and would not be anti-democratic. Most, therefore, have always accepted the principle of consent, although hoping it would never lead to a change in the status quo.
The last groups, then, to accept this principle, have been on the Irish republican side. Surely the will of the people, you might say, is central to the Republican ideal? Well, yes and no. The problem comes down to of which grouping of people do you accept the will? Irish republicans have traditionally claimed that it doesn't matter what the will of the majority in Northern Ireland is, because the majority on the island of Ireland want a united, independent, 32 county republic. How do they know this? We voted for it in 1918, by backing Sinn Féin in that year's general election.
This created a thorny issue. Because of partition in 1918, there was not likely to be another poll on an all-Ireland basis, so republicans have always harked back to 1918 as the last time the will of the Irish people was expressed. Sinn Féin representatives would stand for election, but would refuse to take their seats, whether in Dáil Éireann or the British House of Commons, and refused to give recognition to any governing body in Ireland other than the defunct Dáil of 1918. The rest of Ireland moved on to enthusiastically embrace independence, however, and sweep the issue of partition under the carpet. Latterly, Sinn Féin has given recognition to the Republic of Ireland, accepting seats in the Dáil (they have one deputy at present), and the IRA avoid conflict with any agents of the state (there has never been a huge amount of sympathy with the modern IRA among ordinary Irish people, but the killing of a Garda always causes a huge groundswell of antipathy towards them).
So, extreme republicans never accepted the formation of the Northern Ireland statelet, and therefore claim to be under no obligation to accept the will of its majority. This has been their justification for the IRA's long and ferocious campaign. After 25 pointless and bloody years of the troubles, however, new thinking was required, and many republicans became presuaded of the need for an agreed Ireland, as advocated by the SDLP. The Good Friday Agreement represented the first time Sinn Féin accepted the principle of consent, this being one of the agreement's key selling points to Unionists.
The clever part of the Agreement was that it would be ratified in polls in both parts of the island. This was the first time since 1918 that all of Ireland voted on the same issue (although the wording was different in each territory), which meant that the 1918 election result could be set aside in favour of the "new dispensation". In 1998, 85% of the people of the island voted for the agreement, which meant that the principle of consent was now enshrined as the legitimate will of the Irish people.
Naturally, there are still those who do not accept this result, for various reasons, but the fact that the main republican group was on-side meant that meaningful engagement could now take place.
So, in a nutshell, consent appears to be the basis on which the Irish Question will now be settled. There will be no change in the position of Northern Ireland within the United Kingdom, short of the devolution of powers which has taken place, until the people of Northern Ireland decide accordingly.