The agreement in question is the London Fisheries Convention, which came into effect in March 1966 and allows its signatories to fish within a belt between six and twelve miles from each others' coasts in which they have "habitually fished". It also defines the rights of each coastal state to regulate this belt, for example to "give effect to internationally agreed measures of conservation".
This move is "an historic step towards delivering a fairer deal for the UK fishing industry" and "part of moves to prepare the UK for the opportunities of leaving the European Union", according to the announcement, and affects "vessels from France, Netherlands, Belgium, Ireland and Germany" which "in 2015... caught an estimated 10,000t of fish, including mackerel and herring, worth an estimated £17m" in the belt around the UK.
Formally the move was presented only as "taking back control" but Fishing News referred to "countries that will lose access to the UK’s 12-mile zone". This was certainly the way it was seen by the Irish fisheries minister Michael Creed, who "reminded the UK that some of these rights were reciprocal, allowing not only the Irish fishing fleet access to parts of the UK 6-12 mile zone, but also the UK fleet to parts of the Irish zone".
Creed also pointed out that these access rights were incorporated into the EU Common Fisheries Policy when Ireland and the UK joined the EU, and Michel Barnier "brushed away this decision as irrelevant to negotiations on the Common Fisheries Policy, which superseded the 1964 agreement". As the March EU Council summit approached, the main concern was whether the Common Fisheries Policy would continue to apply to the UK's waters and fishing fleet during the proposed transition period.
How the supposed dates of Brexit (March 2019) leaving the fisheries convention (July 2019) and the end of transition (December 2020) interact hasn't been addressed anywhere I've found, and if a letter ever was sent it isn't on the UK list of negotiation documents.
