A single platform line-transect survey using distance sampling was carried out off the north coast of Ireland on 9 August 2012. Sea conditions were excellent throughout the survey with 100% of survey effort carried out in sea-state ≤3 and 63.5% in sea-state ≤1. A total of 178 km of track-line was surveyed in 654 minutes (10.9 hrs). Five acoustic events were logged during the survey: three harbour porpoise records and one each of common dolphin and Risso’s dolphin. All acoustic events were recorded concurrent with visual sightings. Thus most (77%) of the harbour porpoise sightings and all minke whale sightings obtained during the survey were not detected acoustically. A total of 33 sightings were recorded comprising 158 individuals among five marine mammal species, which suggests good species diversity in the area at this time of year. These included 11 harbour porpoise sightings, one sighting of a large group of common dolphins, two sightings of Risso’s dolphins and six sightings of single minke whales.