Background: Between 1994 and 2006, the incidence of sexually transmitted infections (STIs) in Ireland has increased by over 300%. Recent literature would suggest that this figure is an underestimation of the true scale of infection. Our objective was to determine the risk factors associated with STI diagnosis in a population with a rapidly increasing STI incidence. Methods: Using diagnostic, demographic and behavioural information from three STI clinics (January 1999 to December 2006), multivariable logistic regression models were used to identify risk factors associated with STI diagnosis. Results: Age, smoking and inconsistent condom use are the dominant risk factors. Males aged 20-24 years and females aged < 20 years being at greatest risk of STI acquisition. Having three or more partners was not associated with an elevated risk of STI diagnosis. At univariate level, homosexuals and bisexuals have a decreased risk of STI acquisition compared with heterosexuals. Rate of consistent condom use was low < 13.3%. Conclusions: Age, condom use and number of sexual partners are important risk factors for STI diagnosis. Contrary to international STI literature, having multiple sexual partners does not increase STI incidence. Age specific behavioural interventions that target increased condom use may be effective in reducing STIs in Ireland. At policy level, a reduction in the taxation on condoms from 13.5 to 5% is needed to lower the prohibitive cost and increase their use.