Groundwater quality analyses were carried out on samples from 262 private sources in the Republic of Ireland during the period from April 2008 to November 2010, with microbial quality assessed by thermotolerant coliform (TTC) presence. Assessment of potential microbial contamination risk factors was undertaken at all sources, and local meteorological data were also acquired. Overall, 28.9% of wells tested positive for TTC, with risk analysis indicating that source type (i.e., borehole or hand-dug well), local bedrock type, local subsoil type, groundwater vulnerability, septic tank setback distance, and 48 h antecedent precipitation were all significantly associated with TTC presence (p < 0.05). A number of source-specific design parameters were also significantly associated with bacterial presence. Hierarchical logistic regression with stepwise parameter entry was used to develop a private well susceptibility model, with the final model exhibiting a mean predictive accuracy of >80% (TTC present or absent) when compared to an independent validation data set. Model hierarchies of primary significance are source design (20%), septic tank location (11%), hydrogeological setting (10%), and antecedent 120 h precipitation (2%). Sensitivity analysis shows that the probability of contamination is highly sensitive to septic tank setback distance, with probability increasing linearly with decreases in setback distance. Likewise, contamination probability was shown to increase with increasing antecedent precipitation. Results show that while groundwater vulnerability category is a useful indicator of aquifer susceptibility to contamination, its suitability with regard to source contamination is less clear. The final model illustrates that both localized (well-specific) and generalized (aquifer-specific) contamination mechanisms are involved in contamination events, with localized bypass mechanisms dominant. The susceptibility model developed here could be employed in the appropriate location, design, construction, and operation of private groundwater wells, thereby decreasing the contamination risk, and hence health risk, associated with these sources.