Geomalacus maculosus Allman, 1843 is an EU-protected slug species which is only found in the west of Ireland and northern Iberia. There is little knowledge of its population sizes throughout its range and no long-term studies have been conducted to calculate estimates. Localized populations of G. maculosus and the sympatric slug Lehmannia marginata (Müller, 1774) were monitored over one year using the mark-recapture approach at five different sites within a commercial conifer plantation in Ireland. Slugs were marked using visible-implant elastomers injected into the foot. Up to 23.9 /m2 (G. maculosus) and 5.6 /m2 (L. marginata) specimens were observed during the first survey in August; however, these densities varied greatly in different areas of the plantation and throughout the year. Population-size estimates were also calculated using the Jolly-Seber method. Temperature was significantly correlated with capture success, with fewer captures during the colder months. This is the first long-term study that uses visible-implant elastomers as markers for slugs and we evaluate their use, provide guidelines for trapping intensity, discuss limitations of the mark-recapture method and advise on the optimal survey time and conditions.