The CBS is based on a random stratified approach. The Republic was divided into eight regions, and 10 km squares (based on the Irish National Grid) were randomly selected within each, and allocated in sequence. For each 10 km square selected, the 1 km square at the extreme southwest corner is surveyed. Those with less than 50% land, e.g. coastal areas or lake shores, have been excluded, leaving some 700 possible survey squares. The survey aims to achieve coverage of the same 1 km squares each year, ideally by the same observer, although there is likely to be some changeover of survey participants.
Fieldwork is carried out by a network of BirdWatch Ireland volunteers and National Parks and Wildlife Conservation Rangers. Some 250 participants cover over 300 sample sites (1km squares) across the country each year. The locations of these sites are well distributed across the country.