In the 1930s and late 1950s there were official moves in Ireland to apply a straight baseline system around the Irish coastline. Much internal discussion resulted in Ireland in the postwar period about how the baselines should be drawn. Contemporaneously with such internal official discussion, talks were held with the British authorities. One of the Irish aims was to ensure that as far as possible the UK should approve in advance the proposed baseline system. Despite this, in at least one geographical instance, Ireland ignored the British viewpoint which later led to British diplomatic protest. This disputed part of the Irish straight baselines-and others-may now have to be revised following Irish accession to the new LOS Convention insofar as they infringe international law. This article for the first time analyses recently released Irish archive documents relating to this process.