Under EU regulations, vessels from a non-EU country may only land fish at a limited number of ‘designated’ ports. This is to aid control and compliance and to deter illegal, unreported and unregulated fishing activity.
There are currently two Irish ports that have been designated for landings by vessels from a third country: Killybegs in Donegal and Castletownbere in Cork.
When the UK formally becomes a third country and the Common Fisheries Policy no longer applies to it, UK registered vessels will be restricted to landing fish at those two ports. The vast majority of current UK fish landings are to those ports.
However, if, as I very much hope will be the case, the Withdrawal Agreement is concluded, a transition period will apply where there will be no change to current practices for Irish or UK vessels for at least 2 years.
If, in the worst case scenario, no agreement is reached and there is a disorderly Brexit, then the UK will leave on the 29th of March 2019. In that scenario, UK vessels would be restricted to landing in the two designated ports only from that date.