A party’s designation is decided under section 16C, subsection 3 of the Northern Ireland Act 1998, which states:
For the purposes of sections 16A and 16B and this section, a political party to which one or more members of the Assembly belong is to be taken—
(a) to be of the political designation “Nationalist” if, at the relevant time (see subsection (11)), more than half of the members of the Assembly who belonged to the party were designated Nationalists;
(b) to be of the political designation “Unionist” if, at the relevant time, more than half of the members of the Assembly who belonged to the party were designated Unionists;
(c) otherwise, to be of the political designation “Other”.
So in order for the Alliance party to change its designation to “Unionist”, more than half of its 17 MLAs would have to change their designation first. There are provisions in the Assembly’s standing orders by which a member may change their designation, but they’re pretty limiting - see section 3A subsection 3:
A member may change his or her political designation only if—
- (a) (being a member of a political party) he or she becomes a member of a different political party or he or she ceases to be a member of any political party;
- (b) (not being a member of any political party) he or she becomes a member of a political party.
So the party would have to do some sort of weird juggling whereby they expel and readmit members to allow them to change designation.
However, even if they did this, subsection 11 referenced in the excerpt from the Northern Ireland Act above defines “relevant time”:
In this section “the relevant time” means the end of the day on which the Assembly first met following its election.
So even if the Alliance party MLAs all changed their individual designation mid-term, the party’s overall designation would remain as “Other” until the first meeting of the Assembly after the next election. At this point, it would technically be possible for Alliance to have all its members sign up as unionists, invite the Ulster Unionists to switch party, and join Sinn Féin in government, but realistically this course of events is extremely unlikely.