There is a double taxation agreement between the UK and the Republic of Ireland. Article 17 advises that pensions and lumpsums arising in Ireland and paid to a resident of the uk, are only taxable in the UK.
1976 Ireland/UK Income & Capital Gains Tax Convention
This means that you will need to claim back any tax deducted in Ireland on your pension, as it is not taxable there. You do need to declare this foreign pension / lump sum in a Self Assessment Tax Return.