More than 70 Dublin landlords have been hauled before the courts for renting out substandard accommodation.
Of 4,826 inspections of private rented homes last year, half did not meet regulatory requirements.
A total of 3,039 improvement letters and notices were served against landlords to bring accommodation up to scratch.
Chief Executive of Threshold, Bob Jordan, said: "Dublin city council gives landlords every opportunity possible to improve their property.
"The landlords who are being prosecuted are clearly landlords that are being communicated with many times and simply haven't stepped up to the mark in terms of improving their properties."
The charity is calling for an NCT for rented accommodation where landlords will have to prove their housing meets the basic standards.
