As a worker in ROI you are potentially entitled to exactly the same tax reliefs as a resident worker including the married person's 'tax credit' and mortgage interest relief if you have a mortgage on your home in Newry.  You can claim the latter by completing form TRS1 available on Irish Revenue website, www.revenue.ie.   As a UK resident with earnings that arise outside of the UK you are obliged to complete a return of that income to Revenue in the north under self-assessment rules. You should be given a credit or relief in respect of any Irish income tax you are paying because of the double taxation agreement between the Republic and the UK.  There may or may not be a balance of income tax to pay to Revenue in the north.  Obviously in your case there has been a balance of tax to pay in the north even whilst you have only had a single person's tax credit in ROI.  Please note that whilst you are perfectly entitled to avail of the married person's tax credit in ROI this will of course reduce the amount of ROI tax you pay and thus potentially result in a greater balance income tax liability in the north.