The ground investigation and construction for the North Cut and Cover Section of the Dublin Port Tunnel (north Dublin, Co. Dublin, Ireland) involved high resolution ground investigation and excavation logging. As a result, our interpretation of the glacially derived Quaternary geology encountered presents a more complex picture than previously described. A detailed description of the stratigraphy and lithology of the glacial deposits formerly known as the ‘Dublin Boulder Clay’ is presented, and palaeoenvironmental interpretations are suggested for the various units. The ‘Dublin Boulder Clay’ is reinterpreted to comprise four major units and associated subunits. Possible construction hazards, including discontinuities, lithological variability, glacio-tectonic rafting and the presence of large water-bearing units, are identified and linked with the stratigraphy. Preliminary data on the geotechnical and engineering behaviour of each unit are presented. The encountered lithologies generally demonstrated high in-situ strength and stiffness but low permeability and plasticity.