The population of the Shannonside counties will increase by 21,500 over the next 16 years, according to a new report.
There are around 153,000 people living in Longford, Leitrim and Roscommon at the moment.
The ESRI is projecting that will increase to almost 175,000 by 2040.
It says Leitrim's population will increase by over 6,000, to 41,500, while Roscommon's will grow by almost 11,000, to 81,500, and Longford's will rise by almost 5,000, to nearly 52,000.
ESRI researcher Paul Egan says there will be a need for extra housing to account for the population increase.