Since there is no border control between Northern Ireland and the Republic of Ireland, it shouldn't be a problem if you are traveling by road but make sure you carry your marriage certificate.

If you are flying, she is still entitled to travel with you without visa under the EU directive but that unfortunately is only theory and the check-in staff most probably will not let you board even though I have seen people arguing their way through.

To be on the safe side and avoid any hassle, she can apply for entry clearence as the wife of an EU national which is issued free or charge and quite fast ( can be even the same day ).

Good luck