The UN estimate that 8–10% of all greenhouse gas emissions come from food waste. With the School Meals Programme in Ireland being expanded to every primary school before 2030, the objective of this study was to understand the factors that cause food waste in Irish primary schools, with the aim to manage the problem. Findings from a review of literature were used in designing surveys, for children (answered by parents) and for teachers from 32 primary schools in the Republic of Ireland. The methodology used was an explanatory mixed methods approach with quantitative and qualitative elements in the surveys. Inductive reasoning was carried out with qualitative data to establish sub-themes and themes from the survey response codes. The results show that 88.5% of students consumed half or more of their school lunch. More than 50% of students indicated that lack of time was the reason for their food being wasted whilst the qualitative data from students and teachers revealed other issues relating to the eating environment, menu choices, product related issues, factors relating to parents and children, portion size and the over-arching themes of government and school policies.