Children with acute mental health (MH) concerns increasingly present to Emergency Departments, and in the absence of an accessible MH bed, are admitted. This study estimated the hidden associated costs. All Emergency MH admissions over a 12-month period (2016) were identified (N=105). Costs associated with length of stay (LOS) and one-to-one nursing were calculated. The average Length of Stay for acute MH presentations was 6 days, there were 615 bed day associated with this cohort, totalling costs of €1,216,470, with an average cost/patient of €12,684. Sixty-eight patients (65%) required an average of 5 days of one-to-one nursing, totalling 335 days. Estimating that 40% of this was provided by agency staff, the hospital cost was €115,374. Taken together, the costs associated with primary Emergency Mental Health presentations is €1,331,844. Costs associated with patients who were previously known to MH services were €843,417. Despite an increasing number of dedicated MH beds, demand outweighs availability, and immediate access remains problematic, the default often being a paediatric admission. Adequate funding and appropriate use of these scare and costly resources must be part of national MH policy planning, especially with ongoing planning for the National Children's Hospital.