troubles in Northern Ireland

In Northern Ireland a campaign of shooting and bombing was carried out by IRA, this organisation declared itself to be the legal army of the island of Ireland, in it's constitution it declared itself as the legal government of Ireland, it got it's so called legality from the 1918 all Ireland elections and what they call the first Dail. my question is. Can an organisation which sets it's self up as being the legal government,army of a area of land ie Ireland. could this organisations actions be brought before the International Courts? this organisation declared that both jurisdictions in Ireland illegal, their courts, police everything was illegal, but this organisation only attacked what I understand in international terms as " ethnically, culturally and religiously minority community on the island of Ireland" this community is British, UlsterScots, Protestant. at no time did it allow it's members to attack the Republic of Ireland only Northern Ireland and only the British community, which is a minortiy on the Island of Ireland