Here's a partial description of fascism from Wikipedia:
Viewing the nation as an integrated collective community, they see
pluralism as a dysfunctional aspect of society, and justify a
totalitarian state as a means to represent the nation in its entirety. They advocate the creation of a
single-party state. Fascists reject and resist the autonomy of cultural or ethnic groups who are not considered part of the fascists' nation and who refuse to assimilate or are unable to be assimilated. They consider attempts to create such autonomy as an affront and a threat to the nation. Fascist governments forbid and suppress opposition to the fascist state and the fascist movement. They identify violence and war as actions that create national regeneration, spirit and
vitality.
There's some overlap -- to the best of their ability the IRA violently suppressed opinions different from their own. [And from the tone of your comment, it seems that you don't like opposing opinions, yourself]. The IRA certainly rejected the idea of protestants/unionists having any right to have a meaningful role in Northern Ireland.
It's not a question of being nationalist; it's the IRA's methods that earned it the designation. I'm betting that you don't defend those unionists who resorted to similar tactics.
The 'acting like it was still an empire' line is really lame. The not-very-enthusiastic British government sent troops in when Catholic politicians asked them to (Gerry Fitts, for one, if you need a name), but no government, anywhere, post-imperial or not, could just ignore an armed rebellion that was intended to force itself on a mostly unwilling population.