Huge disparity of scale between the IRA and Taliban.
Taliban: 7,000 to 11,000 (2008 est.)
IRA: 900 to 1,200 (1971 est.)
71' was when the IRA was at its peak other estimates in the 90's are around 500 and today it would be far less.
Estimates vary, my source was wikipedia.
Considering the size difference, Taliban would win easily.
If the IRA consisted of 11,000 soldiers it would be the same size as the Irish Defense Forces!
I could see why the British would be more afraid of the Taliban than the IRA, I would much rather live with the conception of a united Ireland than living under Sharia law. The IRA bomb and shoot their victims, Taliban cut off their opponents heads. Taliban are willing to kill themselves for their cause.
Taliban are more aggressive and determined than the IRA. IRA are better strategists at least against the British.