From Wikipedia

Loyalists Part 3 - War And Peace

Upon the partition of Ireland in 1921, six of the nine counties in the province of Ulster were excluded in the independent Irish Free State (later the Republic of Ireland). These counties, four out of the six having Protestant majorities, remained a part of the United Kingdom. Two other Ulster counties also remained part of the UK, despite having nationalist (pro-independence) majorities. Both unionist and nationalist communities have allowed or encouraged sectarianism among Protestants (associated with unionism) and Roman Catholics (associated with nationalism) to further political aims.

The difference of opinion between Northern Ireland's Nationalist Catholic population (which mostly supports leaving the UK in favour of uniting with the Republic of Ireland) and its Protestant Unionist population (which mainly supports remaining as part of the United Kingdom) has led to a long-running bloody conflict known as The Troubles. However, the majority of people who live in the region do not support paramilitaries of any ideology.