BACKGROUND: Little is known about the prevalence and determinants of drug use among men who have sex with men (MSM) in Ireland. The aims of this study were to measure the prevalence of recreational drug use among MSM in a national sample, and to identify sub-groups of MSM who may benefit from targeted preventive interventions. METHODS: The MSM Internet Survey Ireland (MISI) 2015 was a community-recruited, nationally-promoted, self-completed online survey for MSM. MISI 2015 included standardised questions on recreational drugs, poppers, and drugs associated with chemsex (i.e. crystal methamphetamine, GBL/GHB, mephedrone, ketamine). Multivariable-adjusted logistic regression was used to identify factors associated with use of these substances. RESULTS: In the previous year, 36% of MSM used recreational drugs, 33% used poppers, and 7% used drugs associated with chemsex. Five percent were diagnosed HIV-positive. Recreational drug users were significantly younger than non-users (median = 27 vs. 32 years; p < 0.001); popper users were significantly older than non-users (median = 34 vs. 28 years; p < 0.001). The odds of recreational drug use were higher among MSM diagnosed HIV-positive (vs. never tested; AOR 2.27, 95%CI 1.39-3.70). Use of poppers, and use of drugs associated with chemsex, were also higher among MSM diagnosed HIV-positive (vs. never tested; AOR 3.77, 95%CI 2.41-5.90, and AOR 5.87, 95%CI 3.08-11.18 respectively). CONCLUSIONS: The prevalence of recreational drug use is higher among MSM than in the general population in Ireland, and it is particularly high among MSM diagnosed HIV-positive. Targeted harm reduction messages and preventive interventions are warranted to complement population-based approaches to reducing drug use in this population.