A couple of weeks ago I was at a meeting where the issue of Work Permits for UK workers in Ireland was broached. The general concenus was that UK citizens will need a work permit to work in Ireland post Brexit.
However the current Withdrawal Agreement allows for the status quo to remain intact. Here is an extract from Lexology news feed;
"During the transition period, UK nationals and EU citizens will continue to have the right to take up employment and carry out an economic activity as a self-employed person and will keep all their workers’ rights based on Union law. A person covered by the Withdrawal Agreement who has had his or her professional qualifications recognised in the country where he or she resides or works will be able to continue to rely on the recognition decision in that country. Frontier workers and frontier self-employed workers (i.e. those who are currently nationals of one Member State, reside in another Member State and carry out work in a third Member State) are also protected under the Agreement. This will be particularly welcome news for Irish employers in the vicinity of the border employing EU citizens from countries other than the UK and Ireland where those citizens reside in Northern Ireland"