Despite the positive relationship between science self-efficacy and science motivation, and the prevalence of scientist-led outreach programs, there is limited research on the effect of scientists on children’s science self-efficacy beliefs. Furthermore, there is no extant science self-efficacy research on primary pupils in Ireland, possibly due to the absence of a suitable and valid assessment instrument. This multi-study research aimed to develop and validate a questionnaire suitable for investigating the effect of scientist-facilitated science outreach on the science self-efficacy of children aged 11-12 years old. In Study 1 the first version of the Irish Science Self-Efficacy Children’s Questionnaire (IS-SEC-Q) was developed and tested with 92 primary school students. In Study 2, the revised questionnaire was re-tested with 282 students. For both studies, construct validity was examined through factor analysis. Questionnaire interpretation and comprehension were investigated via interviews (N=4 and 25 respectively). In Study 2, convergent and criterion validity of the scales was also examined. The final IS-SEC-Q contained 5 scales (63 items), including an adaptation of Usher and Pajares ‘Sources of Self-Efficacy in Mathematics’ scale. Study 2 interviews indicated possible misinterpretations of the Emotional State subscale. The questionnaire demonstrated good psychometric properties and should serve well for informal science education practitioners endeavouring to assess their impact on primary children’s science self-efficacy.