The disproportionately high prevalence of tobacco use among prisoners remains an important public health issue. While Ireland has well-established legislative bans on smoking in public places, these do not apply in prisons. This study evaluates a multi-component tobacco control intervention in a medium security prison for adult males in Ireland. A stop-smoking intervention, targeting staff and prisoners, was designed, implemented, and evaluated with a before-and-after study. Analysis was conducted using McNemar's test for paired binary data, Wilcoxon signed rank test for ordinal data, and paired T-tests for continuous normal data. Pre-intervention, 44.3% ( = 58) of the study population were current smokers, consisting of 60.7% of prisoners ( = 51) and 15.9% of staff ( = 7). Post-intervention, 45.1% of prisoners ( = 23/51) and 100% of staff ( = 7/7) who identified as current smokers pre-intervention reported abstinence from smoking. Among non-smokers, the proportion reporting being exposed to someone else's cigarette smoke while being a resident or working in the unit decreased from 69.4% ( = 50/72) pre-intervention to 27.8% ( = 20/72) post-intervention ( < 0.001). This multicomponent intervention resulted in high abstinence rates, had high acceptability among both staff and prisoners, and was associated with wider health benefits across the prison setting.