Higher dietary energy density (DED) has been reported to be associated with weight gain, obesity and poorer dietary quality, yet nationally representative estimates that would allow tracking of secular trends and inter-country comparisons are limited. The aims of the present study were to calculate DED estimates for the Irish population and to identify dietary determinants of DED. Weighed/semi-weighed food records from three cross-sectional surveys (the National Children's Food Survey, the National Teens’ Food Survey and the National Adult Nutrition Survey) were collated to estimate habitual dietary intakes for a nationally representative sample of the Irish population, aged 5–90 years (n 2535). DED estimates, calculated using the total diet method, the food only method and a novel method, including foods and solids in beverages, were 3·70 (sd 1·09), 7·58 (sd 1·72) and 8·40 (sd 1·88) kJ/g, respectively. Determinants of DED did not vary by the calculation method used. Variation in the intakes of fruit, vegetables and sugar-sweetened beverages (SSB) across consumer groups contributed to the largest variance in DED estimates, followed by variation in the intakes of potatoes, fresh meat, bread, chips, ready-to-eat breakfast cereals, and confectionery. DED estimates were inversely associated with age group and consistently lower for females than for males. The inverse association of DED with age group was explained by higher intakes of vegetables, fruit, fish, potatoes, fresh meat and brown bread and lower intakes of SSB, chocolate confectionery, ready-to-eat breakfast cereals and savoury snacks in older age groups. Females consumed, on average, 1·5 times more fruit and vegetables combined when compared with males, largely explaining the sex differences in DED estimates. Current DED estimates for adults were similar to those calculated in a previous survey, carried out 10 years earlier. These estimates and determinants serve as a baseline for comparison for other works and public health campaigns.