I completed Form 12A Application for Certificate of Tax Credits and ticked the Married Persons Tax Credit option, also put my wife's work details on the form (because this was requested). Didn't realise this could impact on her however. I don't see why she should be taxed, in fact when I told her she was quite angry that she could be taxed twice. Is this really the case? I can't believe it, because she already pays tax and national insurance, and has never paid any money to Ireland before, only the UK.