New research has found that around 70% of long-term care patients are taking one or more potentially inappropriate medicines.
It says this has serious implications for older people since adverse drug reactions can occur, leading to falls and fractures, and patients may require extended hospitalisation.
The main drug involved in inappropriate prescribing was the sedative benzodiazepine.
The research involved examining a sample of 630 patients in Northern Ireland and in the Cork area and was funded by the Centre for Ageing Research and Development in Ireland.
Half of the patients in the Republic were prescribed between nine and 14 medicines, but in an extreme case one resident was prescribed 25 medicines.
In Northern Ireland and the Republic, residents of long-term care facilities were frequently prescribed 11 medicines and often took an average of eight medicines daily.
The researchers found that in the Republic older people use four times more medicines than other age groups.
The report says the cost of inappropriate medicines for the 630 residents was €165,513 a year.
The Department of Health and Children has said it is receiving increasing evidence of inappropriate use of benzodiazepines.
It plans to create an offence of unauthorised possession of benzodiazepine and may also introduce extra legislative controls on how the drug is prescribed.
Age Action Ireland has described the findings of the report as 'disturbing'.
Eamon Timmins, head of advocacy, said the Minister for Health must act on the report.