One in four married couples are now living with an adult child; who is classed as someone aged 20 or over.
The census stats analysed by Savills Ireland show children are taking longer to fly the nest because of the surge in rents.
It also points to what's been dubbed the 'boomerang generation' i.e. millennials who had moved out but who are now back home as they try to save for a hefty mortgage deposit.
Director of Research Dr John McCartney says "despite an overall decline of 87,282 in the number of twenty-somethings since 2011, the number of couples living with children aged 20 and over has risen.This trend is particularly evident in less affluent areas where family money may not be available to help young adults onto the housing ladder.”
The phenomenon itself is having an affect on the housing supply because it means older people aren't downsizing and their family homes are not being put on the market.