Background: Injecting drug use (IDU) is a major driver of the European hepatitis C virus (HCV) epidemic.  National data on prevalence of HCV amongst Irish drug users remains confined to certain treatment sites and prison settings. Aim: To examine the prevalence of HCV infection and risk factors associated with infection among the 228 patients attending Opioid Substitution Treatment (OST) in a clinic in Dublin. Materials and Methods: A retrospective cross-sectional study was conducted using data collected from Health Research Board (HRB) forms and standardised written and electronic assessment forms routinely completed on OST initiation. Results: The prevalence of HCV infection was 63.6 % (n= 145) with no significant gender difference (p=0.717). Patients who were infected with HCV were older than those uninfected (41.1 ± 7.5 years versus 37.5 ± 8.5 years; p = 0.001), with prevalence significantly lower in younger adults (p=0.002). Multivariate analysis identified age of first drug use (p=0.002) and first injection (p=0.001), type of first drug used; cannabis (p=0.015), heroin (p=0.014) and cocaine (p=0.018) and early age of OST entry (p=0.001) as the most significant risk factors for HCV infection in this cohort. Those with no IDU had decreased odds of being HCV positive by 91.1%. Conclusion:  Data for this Irish sample indicates high prevalence of HCV infection, and the need to consider age of first drug onset and injecting use, particular drug types and earlier commencement of OST to inform targeted HCV treatment and prevention interventions in Ireland.