The Northern Ireland Assembly Bill, resulting from the White Paper, entered into force on 3 May 1973 and elections to the new Assembly were held on 28 June. The agreement was supported by the nationalist Social Democratic and Labour Party (SDLP), the Unionist UUP and the Inter-Community Alliance Party. The pro-deal parties won a clear majority of seats (52 to 26), but a significant minority in the Ulster Unionist Party opposed the deal. Faulkner found this fact very difficult to sell to anti-Sunningdale trade unionists, believing that the symbolism of the council`s existence replaced all the real powers it was supposed to exercise. The importance of the Loyalists` view of the Council can be understood by their distrust of the motives of the British and Irish Governments. In the latter case, the controversy over Articles 2 and 3 of the Irish Constitution dominated, which, in paper form, confirmed the Irish government`s claim to the six counties as „national territory”, and unionists saw it as a direct attack on Northern Ireland`s status within the United Kingdom, those who opposed Sunningdale were dismayed that the Faulklerian Unionists could accept such conditions. Don Anderson notes how problematic the compromise process has been in Northern Irish politics, that instead of the British method of using compromise as an important negotiating tool, „in Ireland, especially among Northern Ireland loyalists, has connotations of treason.”  Sunningdale was by no means the first or last example of the spread of this attitude, the unionists` reluctance to compromise going back to the home rule issue of the early twentieth century. The 9. A statement was issued in December announcing the deal, which later became known as the Sunningdale Agreement.
In March 1974, pro-agreement trade unionists withdrew their support for the agreement and called on the Republic of Ireland to first delete Articles 2 and 3 of its Constitution (these articles would not be revised until the 1998 Good Friday Agreement). The Sunningdale Agreement was an attempt to establish a Northern Irish power-sharing executive and a cross-border council of Ireland. The agreement was signed on 9 December 1973 at Sunningdale Park in Sunningdale, Berkshire.  Unionist opposition, violence and a loyalist general strike led to the failure of the agreement in May 1974. The 1998 Good Friday Agreement (GFA), on which Northern Ireland`s current system of decentralisation is based, is very similar to the Sunningdale Agreement.  Irish politician Séamus Mallon, who participated in the negotiations, described the deal as „Sunningdale for slow learners.” This claim has been criticized by political scientists such as Richard Wilford and Stefan Wolff. The first said that „he.. significant differences between them [Sunningdale and Belfast], both in terms of the content and circumstances of their negotiations, implementation and functioning`.  There were provisions for a Council of Ireland in the Government of Ireland Act 1920, but these had never been enacted. Unionists did not appreciate any „interference” by the Republic of Ireland in its newly formed region. Following the conclusion of an agreement in 1973 on the formation of an executive, agreement was sought on the re-establishment of a Council of Ireland to promote cooperation with the Republic of Ireland.
Zwischen dem 6. und 9. Talks between British Prime Minister Edward Heath, Irish Taoiseach Liam Cosgrave and the three pro-deal parties took place in the Berkshire city of Sunningdale in December. .