The IRA were not religious radicals. You had a Protestant population whose loyalties lay with the government of the UK and a catholic one whose loyalties lay with the Irish Republic, but this was nothing more than a side effect of Britain adopting a different state religion several centureis ago. Nobody ever solicited or repeated IRA opinions on religious dogma, because they didn't have any. For that matter the IRA wasn't in the habit of bombing Protestant churches in Northern Ireland, but generally attacked military bases and police stations. One Catholic priest was directly involved with the IRA bombing campaign, but this was exceptional, out of hundreds of churches in the region.

The Taliban have a clear military objective too: remove the infidels and their supporters from the country that was formerly theirs.

They were stoning and beheading people and generally running Afghanistan as a medieval hellhole when they were in charge of the place without any miitary opposition, back in the 1990s.

The IRA weren't religious radicals, they were nationalists.

My grand-uncle was an IRA man starting in the 1920's, and probably supported them in some way into the 1980's. It is difficult to convey the depth of feeling that folks like my uncle felt. His father was imprisoned for writing things unfavorable to the British. His uncle was deported for sedition. His entire community was impoverished by onerous taxation and discrimination.

The Taliban want to embrace a fundamentalist vision of Islamic law and institute a theocratic government. Removing foreign influences is a rallying cry, but not the core objective -- they dominated the country well after the Soviet withdrawal, and were themselves a product of foreign intervention. (Ie. Pakistan)