Independent Senator and oncologist John Crown has said it is a great scandal that some private health insurers have decided not to provide an approved cancer drug to their patients.
Ipilimumab is available to people treated publicly for malignant melanomas, but not all health insurers will cover the costs.
The drug, which costs between €40,000 to €50,000 per treatment, is available to all VHI patients and to Laya Healthcare patients on a case-by-case basis.
However, Aviva and GloHealth do not cover the drug.
Professor Crown said it was wrong that those paying premiums year after year were being denied access to Ipilimumab, which he said was the best of immune system drugs to date.
He said it is "incredible" that Aviva pays for the drug in the UK, but not in Ireland.
Speaking on RTÉ's Morning Ireland, Prof Crown said it was wrong that "companies that take people's money" and promise them a choice in healthcare, deny them cover "putting them back in the public health system, so that you and me and all of the other taxpayers ... have to pay for drugs that the insurance companies should be paying for".
Prof Crown said Minister for Health James Reilly deserves credit for helping to help steer the drug through the approvals process, when others said it was too expensive.