We investigated the feasibility of using a T-POD, a passive acoustic dolphin detector system, to monitor bottlenose dolphins in the Shannon Estuary, Ireland, from 27 June to 18 August 2005. A v.3 T-POD, logging alternate minutes, was moored in view of an observation site. Land-based theodolite tracking was used to record the position of the closest animal, school size and activity of the closest dolphin school to the T-POD. All cetacean detections on the T-POD synchronous with shore watches in sea states ≤2 were analysed. A total of 94 schools were observed at distances up to 8000 m from the observer. Acoustic detections corresponded well with visual detections, with 82% of the dolphin schools observed within 500 m of the T-POD detected acoustically. The furthest distance dolphins were observed from the T-POD during periods of acoustic detection was 1246 m. Twelve acoustic encounters were logged without corresponding visual detections, four of which may represent false positives. School size did not affect the acoustic detectability of the dolphins, as there was no difference in the size of schools observed with or without corresponding acoustic detections (Kruskal–Wallis, P=0.64). Similarly no relationship was found between acoustic detections and school sizes at different distances to the T-POD (linear regression P=0.5, r2=0.01). Acoustic encounters did not vary in relation to diel patterns (Mann–Whitney, P=0.13) but were related to tidal state (χ2=40.2, P=0.00, df=11) with more encounters logged in the 4 h after high water, probably reflecting prey-related changes in habitat use. T-POD detections correlated well with visual observations and although detection is likely to vary according to T-POD specification, sensitivity and the conditions at the deployment site, the T-POD is a useful tool that has been shown to offer the possibility of continuous monitoring, something that is difficult to achieve with visual methods alone.