Double Taxation Relief Manual: Guidance by country: Ireland: Income from trade
Income derived by a United Kingdom resident from a trade (including farming), profession or vocation carried on wholly in Ireland is chargeable under Case V (ICTA88/S68), requires the tax to be computed on the same basis as if the income had arisen in the United Kingdom. That is, as if it were chargeable under Cases I or II of Schedule D.
The same allowances, deductions and reliefs are to be given as if the income had arisen in the United Kingdom and additionally a deduction may be allowed in respect of the following items
a) any annuity or other annual payment payable out of Ireland income to a person not resident in the United Kingdom,
b) any annual interest payable out of Ireland income to a person not resident in the United Kingdom where payable before 6 April 1982 on a debt incurred on or before 26 March 1974.
Where the trade, profession or vocation is carried on partly in Ireland and partly in the United Kingdom, the profits or income will be chargeable under Cases I or II.