A robust and cost-efficient method for monitoring some of Ireland's more common bat species was devised in 2003 and has been carried out yearly since then. The objective of the scheme is to provide information on bat population trends and distributions in Ireland. Night-time surveys are conducted in July and August, whereby bat vocalizations are recorded from a time expansion bat detector to a minidisc while driving a known route by car at a constant driving speed. Bat vocalizations are subsequently analysed using sound analysis software. In Ireland, the roadside populations of three target species, the common pipistrelle, soprano pipistrelle and Leisler's bat, can be monitored effectively using this car-based method of survey. Power statistics indicate that a decline of high conservation priority of the least frequently encountered species of the three, Leisler's bat, could be detected within 12 years of monitoring if 25, 30 × 30 km survey squares, each with fifteen 1.6 km transects, are surveyed twice annually. The scheme has revealed new data on the relative bat activity distributions across the island of Ireland. Common pipistrelles are more frequent in the south of the country and Leisler's bats are more frequent in the south and east. Analysis of yearly count data indicated an initially increasing trend in the common pipistrelle and Leisler's bat for the first 4 years of the survey, followed by more recent declines. Ireland has a relatively depauperate bat fauna; hence, a potentially greater number of microchiropteran species could be monitored in other regions using this method. The protocol is easily repeatable and does not have to be carried out by bat specialists. In Ireland, it takes c. 300 h of volunteer time annually to carry out this statistically robust monitoring scheme for all three bat species.