Renters feel unable to speak up because they fear retaliation from their landlord, according to housing charity Threshold.
It's publishing research today which has found tenants will put up with "serious breaches" of the law or their tenancy rights before taking action.
The charity spoke to two dozen renters in Dublin, Cork and Galway who have had problems with their tenancies.
Threshold chair Dr Aileen Hayden said the Irish private rental sector is getting worse.
She explained: "Even when our advisers told them they had a really good case, that they shouldn't leave the property because their notice of termination was invalid... the fear that's there would drive a lot of people to moving out anyway."
Dr Hayden said the situation is linked to more people not being able to afford to buy their own home.
She observed: "What you're seeing is a really significant fall-off in people who can access a permanent home.
"It's about real people who live in real situations, who don't really feel this is their home - they're constantly feeling there's no real future for them where they are."