All income is taxable. Some (such as inheritances) allow you a certain amount before it is taxable and you then pay tax at your relevant rate, depending on your income.
If you are resident in Ireland for tax purposes then any UK pension fund that you cash in is liable to tax (as income). Again, my understanding is that Irish law allows you to take 25% as a tax free lump sum and the remainder is taxable.
It surprises me that there are not more questions on the subject, as you say, moneybox, there must be thousands over here who have such pensions.