Active Retirement Ireland is urging the Government to protect the Free Travel Scheme in next week's Budget.
However, former Labour Party TD Mary Upton has said that a small charge should be levied on longer journeys around the country.
Ms Upton also said the scheme must be protected from abuse by a small number of people using counterfeit cards.
Over 1.1 million people are eligible for the scheme, most of them aged 66 and over.
A spokesperson for the Department of Social Protection said the cost of the scheme to the taxpayer has risen by two-thirds over the past decade and now stands at €76 million a year.
Ministers Joan Burton and Leo Varadkar ordered a review of the scheme a number of months ago.
Active Retirement Ireland says any curtailment would damage the health and well-being of older people.
In a statement, CEO Maureen Kavanagh said members are fearful that if free travel is cut, their links to the outside world would be severed.
She noted that it has become popular "in certain sectors to depict older people as well-off individuals" who can easily shoulder more cuts.
She underlined that many older people are extremely vulnerable, and at real risk of social isolation.
As well as pensioners and partners travelling with them, recipients of Disability and Carer's Allowance and of Invalidity Pension are eligible.
The spokesperson for the Department of Social Protection said the scheme provides free travel on the main public and private transport services.
These include road, rail and ferry services provided by semi-state companies as well as services provided by over 90 private transport operators.
She said the majority of these private contractors operate in rural areas.