A landlord cannot generally justify an increase simply by pointing to higher mortgage, insurance or maintenance costs. They must follow the rent-setting rules applying to the tenancy, including any national rent-control or Rent Pressure Zone limits, and serve a valid written rent review notice within the required timeframe. The notice should explain the new rent, the calculation used and any supporting information required by law, such as comparable rents where relevant. Whether an increase is valid ultimately depends on the tenancy’s facts and can be assessed through the RTB dispute resolution process; check the RTB or gov.ie for the current rules.