You start with 1921 and finish when the Troubles stop.
What are the differences, between Loyalists and Unionists, and between Nationalists and Republicans?
In each case, The Barrel of a Gun.
Nationalists supported more rights for Catholics/Northern Ireland united with the rest of Ireland through peaceful, democratic means. The biggest Northern Irish Nationalist party is the Social Democratic and Labour Party. Republicans supported union through any means necessary including violence and terrorism. The biggest Northern Irish Republican party is Sinn Fein and their paramilitary organization, the Provisional IRA.
On the other side, Unionists support remaining united with Great Britain, but don’t support violence. Loyalists, on the other hand, support union with Britain through any means necessary including violence and terrorism, and set up paramilitary “defence organizations”, first to defend against Republican paramilitaries, and then to engage in violence and terrorism themselves against Catholic areas and assassination of Nationalist and Republican politicians.
In this context, it’s more accurate to say that republicans are a subset of nationalists and loyalists are a subset of unionists.
I don’t think identifying as republican a priori implies using “any means necessary” to achieve the goal of a united Ireland. The term “physical force republicanism” exists because there are republicans who have never had any truck with violence of any kind.
Why start in 1921?