Motorists resident in Ireland with a UK, including NI, driving licence are advised to exchange that licence for an Irish driving licence before the UK withdrawal date from the EU. In the event of a no-deal Brexit, the UK will no longer be a Member State and so the UK (including NI) driving licence will not be recognised.
A UK licence that was granted on the exchange of a licence from a third country would not be eligible for exchange for an Irish licence unless we have an exchange agreement in place with that third country.
At present, Ireland has such agreements in place with 5 Canadian provinces, namely Ontario, Manitoba, Newfoundland & Labrador, British Columbia and Saskatchewan. If the person obtained their licence in one of these provinces, their licence can be exchanged for an Irish licence.