Groundwater vulnerability assessment is a key element of any groundwater protection scheme. In Ireland, groundwater vulnerability is determined mainly according to the thickness and permeability of the subsoils (glacial tills and other superficial deposits). The relative permeabilities of the subsoils are assessed qualitatively as high, moderate or low. To improve the robustness of the groundwater protection scheme, research was carried out into subsoil properties with the aims of refining the permeability ratings, and of improving the way in which subsoil permeability classes are assigned. This research focused on subsoils in the low and moderate permeability categories, mainly tills. Important issues investigated were the relationship between permeability and the grain size distribution of the subsoil, description of subsoils for permeability classification, correlation between permeability and indicators of aquifer recharge, and suitable field and laboratory methods for measuring subsoil permeability. A standard system for describing subsoils was selected, namely BS5930:1999, the choice being influenced by the familiarity of this system among the main users of the vulnerability maps. Analysis of subsoil field descriptions and grain size data indicate that those samples identified as ‘CLAY’ on the basis of BS 5930 correspond to the low permeability category, and tend to have more than 13% clay size particles. The permeability values obtained from each method are compared and indicate that the numerical boundary between moderate and low permeability lies in the region of 10−9 m/s. Differences between the results from laboratory and various field permeability test methods can be explained by differences in scale and by the presence of discontinuities. The research was successful in refining the permeability ratings and thereby in making the vulnerability maps more defensible against possible challenges. This research has improved the way permeability maps are produced in Ireland, and may prove useful in other countries where permeability data are scarce and mapping relies largely on field assessment of subsoils.