Abstract The present study explores the variation in the plural inflection of eight varieties of Frisian and English, focusing on irregular plural formations. The aim of the study is to identify and assess the significance of the factors which contributed to the preservation and emergence of irregular plural patterns in the investigated varieties. Multivariate regression analysis, which was employed to weigh the significance of individual factors, reveals that the retention and emergence of irregular plural patterns is primarily determined by a combination of three major factors: relative frequency, absolute frequency and salience. These factors can be reduced to underlying cognitive processes, including the storage and retrieval of linguistic information in the human brain. Another relevant factor, semantics, affects the lemma frequency profile, but can gain momentum as a factor in its own right due to analogical co-activation in language processing.