We have just got this from Brokers Ireland..
In the event of a no deal Brexit, UK licences will be invalid in the Republic of Ireland for Irish residents. UK licence-holders resident in the Republic of Ireland will need to apply for a full Irish licence (as a learner driver) and go through the application process. There are potential implications of not swapping to an Irish licence before 29 March 2019 in a no deal scenario. Implications could include an impact on premium, as the driver would be a “learner” driver and as a learner they would need to be accompanied.
This in turn could impact employment should a full licence be a requirement for a position. There could also be implications for the contract of insurance depending on its terms. UK licence-holders resident in the Republic of Ireland should be advised to exchange their licence NOW, in case there should be a no deal Brexit.