Violations included mould, broken toilets, exposed wiring and rooms without windows
A three-year Dublin City Council investigation into sub-standard flats found 90% of those inspected were below minimum standards.
Over 6,360 flats were inspected between 2012 and 2015, with 5,685 falling short of the council's requirements.
Almost three-quarters of these have since been brought up to standard following enforcement measures, the council says.
Half of those in breach of legislation were failed because of fire safety problems, ranging from exposed wiring to broken fire alarms.
Over 40% had structural problems including cracked walls, broken windows and unsecured doors. One-third had inadequate ventilation, often leading to mould.
Broken toilets, rooms without windows, sub-standard cooking and laundry facilities were among other violations of the council's standards.
Following the Intensified Inspection Programme of Private Rented Houses, enforcement measures resulted in the issuing of prohibition notices to 240 property owners. These prohibit the rental of a private dwelling until the council is satisfied it is compliant with legislation.
Legal action was also brought against 103 landlords.
The programme focused on pre-1963 flats on 39 streets in the city. In 1963, legislation was introduced that prevented the splitting of houses into flats and bedsits without planning permission.
The areas inspected include Harold's Cross, Rathmines, Fairview, Drumcondra, Phibsboro and other locations in the inner city.