I can't speak to reasons, but Scotland was also very resistant to reformation. If someone answers, it'd be interesting to hear about them both.

Of course, reluctance towards the reformation was widespread across the British Isles, the Pilgrimage of Grace being a prime example. However, it ought to be noted that forcing a new prayer book and faith on your subjects is easier when you can directly control them. The Monarch found it much easier to change the views of people when they were pacified and easily accessible - which the English tended to be much more so than the Irish across the North Sea.

I would also suggest that the Irish were resistant to Ulster plantations and settlers, who were overwhelmingly Protestant. As such Catholic faith becomes as much a symbol of resistance against foreign rule as speaking Irish Gaelic.

A third aspect is that Protestantism tended to be a product of literate, established thinkers, for the peasantry, who had been told all their lives that the only way to get to heaven was through the Church of Rome, converting to a new church was much more of a leap to demand.