OBJECTIVES: MSM Internet Survey Ireland (MISI) 2015 was an anonymous, self-completed, cross-sectional internet survey assessing sexual behaviours and health needs among men who have sex with men (MSM) in Ireland. We explored factors associated with self-reported STI diagnosis among MSM who were sexually active and had an STI test in the previous year. METHODS: We compared the study population (n=1158; 37% of total population), with the sexually active MISI population not testing for STIs (n=1620; 52% of total population). Within the study population, we identified sociodemographics and sexual behaviours associated with self-reporting STI diagnosis. We used multivariable logistic regression to estimate adjusted odds ratios (aORs). RESULTS: The sociodemographics, lifestyle and sexual behaviours of the study population differed significantly from the sexually active MISI population who did not test for STIs. Within the study population, 65% met a sexual partner via geosocial networking smartphone application (GSNa) and 21% self-reported an STI diagnosis in the previous year. On univariable analysis, factors associated with STI diagnosis included: older age, identifying as gay, HIV-positive status, increasing number of sexual partners in the previous year, condomless anal intercourse (CAI) with ≥2 non-steady partners and using GSNa to meet a new sexual partner in the previous year or most recent sexual partner. On multivariable analysis, STI diagnosis was associated with: being aged 25-39 years (aOR 1.8, 95% CI 1.04 to 3.15), CAI with ≥2 non-steady partners (aOR 2.8, 95% CI 1.84 to 4.34), total number of sexual partners (aOR 1.02, 95% CI 1.00 to 1.03) and using GSNa to meet a new sexual partner (aOR 1.95, 95% CI 1.12 to 3.39). CONCLUSIONS: STI diagnosis among MSM testing for STIs is associated with GSNa use, as well as sexual behaviours. GSNas are key settings for STI prevention interventions, which should prioritise men with high numbers of sexual partners and those with multiple CAI partners.