OBJECTIVE: To estimate the additional impact of dementia on in-patient length of stay (LOS) and related costs in Irish acute hospitals. Both principal and secondary diagnosis effects are estimated and valued. METHODS: This is a cross-sectional study based on administrative data collected on all public hospital in-patient discharges in Ireland for people aged 65 years and older in 2019. Coarsened exact matching (CEM) was undertaken to account for observed confounders between dementia and non-dementia groups, while generalised linear modelling (GLM) was used to compare differences in LOS. RESULTS: Patients with a principal diagnosis of dementia spent on average 17.5 (CI: 15.42, 19.56; p < .01) d longer in hospital than similar patients with no principal diagnosis of dementia. LOS was 6.7 (CI: 6.31, 7.14; p < .01) d longer for patients with a secondary diagnosis of dementia compared to similar patients with no secondary diagnosis of dementia. The additional annual cost of care for patients in hospitals with a secondary (principal) diagnosis of dementia was €62.0 million (€13.2 million). CONCLUSIONS: This study highlights the economic impact of extended LOS for patients with dementia in Irish acute hospitals. Addressing specific dementia-related needs of people in hospital is likely to optimise resource use and decrease health care costs in acute care settings.