Less than a third of tenants are renting because they choose to, according to a new survey by Threshold.
The housing charity says seven in 10 renters are doing so because they cannot get a mortgage or qualify for social housing.
45% of the 300 polled spend nearly a third of their take-home pay on rent.
Threshold Chief Executive John-Mark McCafferty says that is of great concern.
He said: "Worryingly one in seven are saying that they spend more than half of their earnings on rent.
"And given that if something else happens in your life - you lose your job or your hours are reduced, or there are additional costs such as a child being born into the family - that might mean the difference between holding on to your home and facing homelessness."
Some 70% of tenants have been renting for five years or longer, with 44% still expecting to be renting in five years’ time.
Our survey of tenants reveals that 14% of tenants are spending more than half of their take-home pay on rent. Renting is increasingly becoming out of reach for many people. Find out more at https://t.co/cfJZwXzNrE pic.twitter.com/AzQ1iwdYB2— Threshold (@ThresholdIRE) June 11, 2018
And 47% of tenants said they felt insecure in their tenancy - with 31% having experienced a rent increase in the previous 12 months.
Of those, 65% said the rent had been increased in excess of the 4% Rent Pressure Zone cap.
Threshold Chief Executive, John-Mark McCafferty, said:"It is extremely worrying, but not surprising, that almost all of those surveyed said they had difficulty in finding rental accommodation.
"Nearly half of those surveyed were working, but the fact that 14% of tenants are spending more than half of their take home pay on rent shows that renting is increasingly becoming out of reach for many people.
"That, coupled with the fact that only 28% of those polled expect to be in a position to own their own home in five years’ time begs the question – what is the long-term solution for this group of people?".
Renting is increasingly becoming out of reach for many people. What is the long-term solution for this group of people asks our CEO @JMMcCafferty. More at https://t.co/b98widxbDc pic.twitter.com/57O9mz9n9u— Threshold (@ThresholdIRE) June 11, 2018
"At least one-in-five of all Irish households now live in the private rented sector - close to the European median.
"While our tenancy protections have improved over the last 15 years, legislative measures to protect renters are not robust as those in most other European countries and are failing to protect many vulnerable tenants in a market where there is an acute shortage of supply, and ever-rising rent costs."
It comes as Threshold is marking its 40th anniversary this year, with a series of events throughout the year - including a policy event in the autumn.
Between 1978 and 1980, the charity says it dealt with in the region of 3,000 cases - while in the first three months of 2018, it had already helped 3,640 clients.
Mr McCafferty added: "The private rented sector, upon which we are so reliant, is broken.
"And while we welcome the Government’s initiatives to improve the situation, regulation alone is not enough.
"Enforcement and real penalties are required for the legislation to become effective."