Update 10.38am: Landlords are blaming the bedsit ban for overcrowding in rental properties.
Prime Times investigators have uncovered dozens of tenants living in cramped conditions throughout the capital.
They've also revealed that less than 4% of homes were inspected last year - most of which were substandard.
Fintan McNamara, the Director of the Residential Landlords Association, said bedsits are a simple solution.
"The quality and standard of accommodation in this country is extremely high now. It is at an all-time high. There are situations where, because of the housing crisis, there is overcrowding - and because we have a huge shortage," he said.
"One of the reasons we have such a shortage is the blanket ban on bedsits. It was a huge mistake to do that. There's still 2,000 of them there and landlords are not allowed to let them out," Mr McNamara added.
Earlier: An investigation into dangerously overcrowded private rental accommodation in Ireland has shown more than two thirds of rental properties in Ireland are failing to meet national inspection standards.
RTÉ Investigates' 'Nightmare to Let', which was six months in the making, showed that in some counties 100% of properties failed inspections.
Three multiple occupancy buildings located in Crumlin, Kilmainham and Rathmines, with more than 120 tenants between them have since been closed after inspection by Dublin Fire Brigade.
One building alone contained over 60 tenants while another had over 40 tenants.
Figures supplied to RTÉ Investigates under Freedom of information reveal that only 4% of the 325,000 registered rental properties in Ireland were inspected last year and of those that were inspected, more than two-thirds were not compliant with the regulations.
It also said some rental properties were not registered and therefore never inspected.
There was a 100% failure rate in four districts - Kilkenny, Louth, Offaly and Limerick City and County, according to RTÉ.
The record was almost as bad in Clare at 99%, Carlow 98%, Galway County 97%, Meath 90%, South Dublin 88% and Sligo 88%.
The figures show that the overall non-compliance rate for the country stands at 69%.
An undercover RTÉ reporter rented one of 40 beds across 10 bedrooms in a property on the Old County Road in Crumlin - for €250 a month.
The programme found the building did not have proper firefighting equipment or proper fire detection systems.
An extension was added to the property bringing the total number of tenants to 64.
The owner of the property, Andrew O'Neill owned five other rental properties which RTÉ said gave him a potential income of €26,000.
The building did not have a properly maintained fire detection system or properly maintained firefighting equipment.
RTÉ said the reporters' fire concerns were reported to Dublin City Council four times before Dublin Fire Brigade inspected the property and order an evacuation of the building which a High Court judge described as a "fire trap".
#RTEInvestigates: 'The accommodation I was in was the horriblest place I've ever been ... I was at the lowest point in my life being there.' pic.twitter.com/SC738YwGof— RTÉ News (@rtenews) November 2, 2017
The programme also highlighted the story of one man who lived in a multi-occupancy dwelling in Dublin for 20 years who said he was unable to use the shower for 504 days.
The investigation discovered Dublin City Council initiated legal action in just 1% of 1,400 properties which were non-compliant with regulations.
Further to this, Kildare was the only other council to pursue legal action, by pursuing two cases.
Robin Knox, a fire safety expert, said the conditions shown in the programme were the worst he had ever seen.
"This takes the biscuit. This is the worst I've ever seen. Sixteen beds in one room - words can't explain how potentially dangerous this is.
Jan O'Sullivan - Labour's Housing spokesperson - said the investigation exposes a sordid and exploitative underground black market in the rental sector that needs to be stamped out.
"We cannot allow it to continue so there has to be the resources given to the local authorities to ensure that there is proper inspection," she said.
"I believe there is potentially possible criminal investigations here because people may well have had their lives put at risk," she added.
Aideen Hayden from the housing charity Threshold said the situation is becoming significantly worse and its a failure of regulation.
"Day in and day out we see people living in appalling circumstances. Not just overcrowding but also the most appalling situations: damp, mould and properties you wouldn't and couldn't possibly bring up children in," she said.
"At the end of the day unless we enforce the legislation we have which we are not doing, we are not protecting people, we are not protecting their rights and we're failing them," she added.
The investigation also featured a commercial property in Kilmainham where tenants were charged €350 a month.
The 42 tenants of the Global Academics-owned property were found to be living in a potentially dangerous building under the Fire Services Act 1981.
The company, owned by Jason Orr and Joshua Cantwell, was forced to close the premises under a fire safety notice.
The programme also showed a three-bedroom home in Rathmines which imposed a 10pm curfew on its 23 tenants.
The home had just one shower and two toilets for which tenants paid €250 a month.
The Junior Housing Minister has said the condition of some rental accommodation is a disgrace.
Damien English said the government is setting aside €2.5m for inspections of accommodation next year.
Minister English wants to see this rise to 25% of accommodation as soon as possible.
"I think it is nothing short of disgraceful. These are rogue landlords, it's as simple as that and they have to be dealt with," he said.
"The legislation is in place to prevent this happening and naturally we need to fund more into inspections.
"Obviously, there is a reporting mechanism there that any tenant or anybody who sees this happening can report it into the authorities.
"It should be dealt with and certainly it will be dealt with," he said.