1. THREE ELEMENTS OF THE USUAL CABINET COALITION BARGAINING
Which parties will be in government?
What (perhaps minimal/incomplete) platform can these parties agree upon?
How will cabinet portfolios be allocated? (The set of (junior) ministries may be flexible)
THREE ELEMENTS OF CABINET COALITION FORMATION
IN NORTHERN IRELAND
The leading unionist and the leading nationalist party will always be in government as the executive and deputy executive.
Cabinet portfolios will be allocated in accord with party vote shares to those parties that agree to be in government using the D’Hondt rule as a sequential allocation process, i.e., the largest party gets to be First Minister, the next largest Deputy First Minister and the next ministry is chosen by whichever party has the largest unallocated divisor quotient. Etc.
There is no overall agreement; roughly speaking, the party that controls a ministry controls the policy within the jurisdiction of that ministry. Ministries are basically fixed in advance.

2. O'Leary, Brendan, Bernard Grofman and Jorgen Elklit.
2005. "Divisor methods for sequential portfolio allocation
in multi-party executive bodies:
Evidence from Northern Ireland and Denmark.”
American Journal of Political Science 49(1),

3. In contrast with Westminster system cabinets, which generally need only be backed by a majority of legislators, ministerial positions in the Northern Ireland Executive are allocated to parties with significant representation in the Assembly. With the exception of justice, the number of ministries to which each party is entitled is determined by the D'Hondt system.
In effect, major parties cannot be excluded from participation in government and power-sharing is enforced by the system. The form of government is therefore known as mandatory coalition as opposed to voluntary coalition where parties negotiate an agreement to share power. The Democratic Unionist Party (DUP), the Ulster Unionist Party (UUP), the Alliance Party of Northern Ireland and some Social Democratic and Labour Party (SDLP) members favour a move towards voluntary coalition in the longer term but this is currently opposed by Sinn Féin.
The Executive cannot function if either of the two largest parties refuse to take part, as these parties are allocated the First Minister and deputy First Minister positions. However, other parties are not required to enter the Executive even if they are entitled to do so; instead, they can choose to go into opposition if they wish. There were some calls for the SDLP and the UUP to enter opposition after the 2007 Assembly elections,[5] but ultimately the two parties chose to take the seats in the Executive to which they were entitled,
On 25 May 2016 a new executive was announced (three weeks after assembly election). For the first time in the assembly's history, parties that were entitled to ministries (i.e. UUP, SDLP and Alliance) chose instead to go into opposition following a recent bill providing parties with this choice. This meant that the executive was formed only by the two major parties, the DUP and Sinn Féin, and thus giving them more seats in the Executive (with the exception of the Department of Justice which was given to an Independent Unionist MLA, Claire Sugden, due to this appointment needing cross-community support).[

4. On 9 May 2016,[3] the number of ministries and departments of the Northern Ireland Executive was reduced, leaving the following departments:
The Executive Office
Minister of Agriculture, Environment and Rural Affairs
Minister for Communities
Minister of Education
Minister for the Economy
Minister of Finance
Minister of Health
Minister for Infrastructure
Minister of Justice