This 20 year dataset contains data on cell abundance of harmful taxa, namely, Dinophysis acuta, Dinophysis acuminata, total Dinophysis spp., “Pseudo-nitzschia delicatissima” complex, “Pseudo-nitzschia seriata” complex, total Pseudo-nitzschia spp., Karenia mikimotoi and total Karenia spp. in the period from 1st January 1997 to 31st December 2016; this time period matches the hindcast numerical model output (20 year hindcast [1997-2016], SW_IRL_ROMS_Hindcast). The geographic boundary box of in-situ sites in this database is located similar to the hindcast model domain, i.e., 11.602° W ; 8.025° W and 50.078° N; 52.788° N. This dataset was created for the use in statistical analyses to facilitate investigation into the relationships between toxin and harmful algal events.