A property management company in Dublin is renting a number of houses across the capital with up to 15 people sharing one room.
The owner of two of the properties implicated said he leases the houses to the company but insisted "there's no breach of regulations".
However, several tenants have revealed to the Irish Independent that they have no written leases.
One house, located in Lehaunstown, Cabinteely, was viewed by this newspaper.
The house has up to 70 people living there, with only two showers in the dwelling.
The basement has been converted into a room which sleeps up to 15 people, and all bedrooms have a minimum of five people in them.
Another room, located on the ground floor, has five sets of bunk beds and is for females only.
The 70 tenants are paying €200 each a month, meaning the company would potentially be collecting around €14,000 in rent - or €168,000 a year.
Another property managed by the company and located in Dundrum has a double room available for €800 a month, and two rooms with four single beds - each bed comes at a price of €350 a month, meaning the landlord would be collecting €3,600 in rent.
Another property managed by the company, located in the Rathmines area, has between 20 and 40 people living there - all females. The owner of the property denied any knowledge of this.
One tenant, Jordan Rhodes (21) from Newcastle, said he lived in the house out of "desperation".
"When I moved in, there was around 40 people living there...When I left, there was around 70 after they put more bunk beds in the basement. There are extension leads all over the house because there are so many people," he said.
The Cabinteely property is not registered with the Residential Tenancies Board.
The company is run by a father and son.
The son has denied any involvement in the properties viewed by the Irish Independent and, that as of two weeks ago, the company has been passed over to new owners.
However, the Companies Registration Office confirmed that no application has been made for a change of directorship.