Concurrent visual and acoustic surveys for cetaceans were carried out in two survey blocks in the Irish Sea to investigate species distribution, relative abundance and absolute abundance where possible. Single platform line-transect surveys were carried out in the northern Irish Sea in July and in the southern Irish Sea in August 2011. During the two surveys, we carried out 348km of survey effort along 23 track-lines of which 100% of the northern Irish Sea survey and 79% of the southern Irish Sea survey were in sea-state ≤3. We recorded a total of 71 cetacean sightings comprising 111 individuals of two species. In addition there were five seal sightings of two species and a single sighting of a basking shark. Harbour porpoise was by far the most abundant species followed by minke whale. Grey seal was the most frequent seal species with only a single sighting of a common seal.