Taxation of Income from clients in Northern Ireland

[Baby boomer]

I'm semi-retired and do a little consultancy / training business as a sideline. Up to now all my business has been done in the 26 counties and is included in my normal tax return. I have now acquired a significant client in Northern Ireland for whom I will run regular training days. Estimated earnings will be about £15k pa. The training will be carried out at the clients premises in Northern Ireland.

Is this income taxable in Ireland, Northern Ireland or both? What about PRSI or National Insurance?