My brother is working as an employee for a company in Northern Ireland. My understanding is that if he only had UK income he does not need to file an Irish tax return due to transborder workers relief. However, he has recently begun earning some additional income from self employment in Ireland although the level of income is relatively small, about €5k p.a. Therefore he has registered as self employed and will have to file an Income Tax return each year with his worldwide income. Is he still exempt from paying additional income tax on the UK employment income under transborder workers relief? Does he also have to pay USC on this UK income now that he has Irish source income on top of the UK income? If he did, it is likely to mean a relatively large additional tax bill in Ireland for a small amount of self employed Irish income. Finally, is he exempt from paying PRSI since he has already paid national insurance in UK on his employment income?