Objective: To describe relationships between the portion sizes of a range of foods commonly consumed by Irish children and adolescents and key indicators of dietary quality on the days they were consumed. Design: Cross-sectional data from the Irish National Children’s Food Survey (2003–2004; 7 d weighed record) and National Teens’ Food Survey (2005–2006; 7 d semi-weighed record) were used to compare mean values for a number of dietary quality indicators (e.g. energy-adjusted intakes of saturated fat, dietary fibre and Na) across portion size tertiles for a range of foods, on the days the foods were consumed. Setting: The Republic of Ireland. Subjects: Nationally representative samples of children aged 5–12 years (n 594) and adolescents aged 13–17 years (n 441). Results: Relationships between food portion sizes and indicators of dietary quality on the days the foods were consumed were similar in both children and adolescents. Lower dietary energy density and saturated fat intakes, and higher dietary fibre intakes, were observed on the days larger portions of fruit and boiled potatoes were consumed. Higher dietary energy density and lower micronutrient intakes were observed on the days larger portions of sugar-sweetened beverages were consumed. Higher Na intakes were observed on the days larger portions of frying meats were consumed. Conclusions: The current work identifies foods for which larger portion sizes may be associated with positive dietary attributes, as well as the opposite. Findings will form an evidence base from which more specific dietary guidance relating to portion size may be developed for Irish children and adolescents.