The purpose of this research was to investigate the effects of Irish medium education (immersion) on children’s first language (L1; English) skills in the educational setting and on their executive functioning (EF) skills. A battery of tests was used to compare 8 YearOld and 12 Year-Old children’s performance on a range of tasks testing their L1 vocabulary, reading, writing, creative, and descriptive (academic) skills and their attention, inhibition, and task switching (EF) skills. Data were collected in two school types (immersion and English medium) in two areas in the Republic of Ireland and one school type (English medium) in one area in Northern Ireland to represent a monolingual sample. As such, this provided a comparison of three school types and, in particular, the effects of learning a second language (L2) to varying degrees of ability—either total immersion (successive bilingualism) or L2 learning for approximately 3.5 hours per week—upon children’s L1 and EF skills. Overall, results revealed that immersion education in Irish had no detrimental effects on children’s L1 (English) or EF skills. Indeed, results suggest that whereas immersion may have helped to enhance children’s attention to and control of their L1, successive bilingualism itself had limited influence on EF skills, although there were some evidence of heightened performance in tasks of attention, inhibition, and task switching. These findings, in addition to the majority of research into bilingualism in the worldwide setting, could be used to inform parents and policy makers that Irish medium education has no negative effects upon children’s L1 skills in the educational setting or their EF skills of attention, control, and inhibition.