There are 895,600 people living in rented accommodation in Ireland and, as a proportion of the overall population at 18.9%, is now the highest since records began.
A report on the private rental sector from estate agent Savills says this figure has risen by 39,500 (4.6%) in the last year.
Savills notes the average number of people in each rental property is also at a record high, rising from 2.7 to 2.8.
The estimated vacancy rate is described as "razor thin" at 1.31%.
Almost a quarter of Dublin's population is made up of renters.
The report author, Savill's Director of Research Dr John McCartney, said the "continued expansion of Ireland's private rented sector should not come as a surprise.
"Property prices continue to advance faster than average earnings, creating an affordability challenge to home-ownership, which is driving people into the rented sector."
For rents to stabilise, Savills says the vacancy rate would need to be 5.6%, which it says is not likely to happen in the forseeable future, meaning rents are set to continue to rise.
The research also indicates cash investors are now "crowding out" mortgage financed buy-to-let landlords, with over 60% (c. 200,000 units) of the country's private rental stock owned outright by investors.