As a married person you can opt for the single or the married tax allowance in ROI, residence is irrelevant. This is known as the 'married person's tax credit' and would increase your take home pay. Of course it would also increase any balance of tax payable to HM Revenue in NI as you would then be paying less Irish income tax. In terms of workers with no NI tax to pay they are either not declaring their ROI earnings or they are declaring but are paying Irish tax which is the same as or greater than NI/UK tax on equivalent earnings in NI.