This study focuses on the dispersion of the Eyjafjallajokull volcanic ash plume over the west of Ireland, at the Mace Head Supersite, and its influence on cloud formation and microphysics during one significant event spanning May 16th and May 17th, 2010. Ground-based remote sensing of cloud microphysics was performed using a K-a-band Doppler cloud RADAR, a LIDAR-ceilometer and a multi-channel microwave-radiometer combined with the synergistic analysis scheme SYRSOC (Synergistic Remote Sensing Of Cloud). For this case study of volcanic aerosol interaction with clouds, cloud droplet number concentration (CDNC), liquid water content (LWC), and droplet effective radius (r(eff)) and the relative dispersion were retrieved. A unique cloud type formed over Mace Head characterized by layer-averaged maximum, mean and standard deviation values of the CDNC, r(eff) and LWC: N-max = 948 cm(-3), (N) over bar = 297 cm(-3), sigma(N) = 250 cm(-3), r(eff) max = 35.5 mu m, (r(eff)) over bar = 4.8 mu m, sigma(reff) = 4.4 mu m, LWCmax = 0.23 g m(-3), (LWC) over bar = 0.055 g m(-3), sigma(LWC) = 0.054 g m(-3), respectively. The high CDNC, for marine clean air, were associated with large accumulation mode diameter (395 nm) and a hygroscopic growth factor consistent with sulphuric acid aerosol, despite being almost exclusively internally mixed in submicron sizes. Additionally, the Condensation Nuclei (CN, d > 10 nm) to Cloud Condensation Nuclei (CCN) ratio, CCN:CN similar to 1 at the moderately low supersaturation of 0.25%. This case study illustrates the influence of volcanic aerosols on cloud formation and microphysics and shows that volcanic aerosol can be an efficient CCN. (C) 2011 Elsevier Ltd. All rights reserved.