Ryanair buying 25 new-build houses in Swords, Co Dublin, is another "kick in the teeth" for first-time buyers, according to Social Democrats housing spokesperson Cian O'Callaghan.
The airline has bought the properties to rent as cabin crew accommodation.
Ryanair said: "In recent years the absence of affordable rental accommodation has been a major impediment to recruiting and training new Irish and European cabin crew members to Ryanair's in-flight team."
Mr O'Callaghan said people are shocked Ryanair has entered the residential property market.
"It is already hard enough for people looking to get a place of their own when rents are at the highest ever level and house prices are 6% above their Celtic Tiger peak," he said.
"Add to that, having to bid against international investment funds and global companies, and it is no wonder there over half a million adults stuck living in their childhood bedrooms.
"This Government is presiding over a dysfunctional housing system that rolls over for giant corporations but offers little hope to ordinary people.
"What will happen to supply for first-time buyers if other large companies follow Ryanair’s example?"
A spokesperson for Ryanair said: "Ryanair confirms that it has purchased 25 new built units in Fosterstown, Swords, in order to ensure that it can provide high-quality but affordable rentals close to Dublin Airport for new cabin crew who are joining Ryanair based in Dublin, to help us maintain Ryanair's flight schedules for up to 33 Dublin aircraft.
"This accommodation, which is located one bus stop from Dublin Airport, will be rented at affordable rates to Ryanair cabin crew during their first year of employment."
In 2021 the Government introduced a 10% stamp duty on bulk purchases over over ten properties.
New guidelines requiring Local Authorities and An Bord Pleanála to prohibit bulk purchases on houses and duplexes in future planning applications were also introduced
Around 40,000 houses and duplexes granted in applications between May 2021 and last December have had this condition attached, a Government spokesperson said.
They said that there are a "small number of legacy instances" where bulk purchases have happened since, due to permissions on the developments being granted before the new rules came into effect.
