Purpose <p>To describe the prevalence of molar incisor hypomineralisation (MIH) and a selection of potentially associated factors amongst 8-year-olds in Ireland.</p> Methods <p>This study reports cross-sectional data from the Fluoride and Caring for Children’s Teeth (FACCT) study 2017. Eight-year-olds in Dublin (<i>n</i> = 786) and Cork-Kerry (<i>n</i> = 1524) were clinically examined for MIH (EAPD criteria) and dental caries (DMFT). The association between potential aetiological factors and MIH was assessed using multivariable logistic regression. Potential effects of MIH on dental caries, oral health-related quality of life (OHRQoL) and parental perceptions of the appearance of their child’s permanent incisors were investigated.</p> Results <p>MIH prevalence was 11% in Dublin and 9% in Cork-Kerry. In Dublin, prevalence of MIH was higher amongst children who had health problems in the past year. There was no association between community water fluoridation and MIH in Cork-Kerry. In both regions, dental caries was higher amongst children with MIH. Parents of children with MIH in Dublin and Cork-Kerry were more likely to have noticed marks on their child’s permanent incisors. In Dublin, MIH was associated with poorer parent-reported OHRQoL. Parents of children with MIH in Cork-Kerry were less likely to be satisfied with the colour of their child’s permanent incisors.</p> Conclusions <p>MIH prevalence was 9–11%. The association between MIH and dental caries suggests that MIH severity in Ireland may be high. There is an urgent need for national data on MIH prevalence and severity, early identification of children with MIH and provision of preventive and treatment services in line with international best practise.</p>