From Wikipedia
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.