A landlord generally must give at least 90 days’ notice of a rent increase, and rent can usually be reviewed only once every 12 months. The written notice should state the new rent, the date it will take effect and the basis for the increase, including any required comparable rents or calculation. Current rent-control rules may limit the increase, depending on the tenancy and its circumstances. If you believe the notice is invalid or the increase is unlawful, you can refer the issue to the RTB; check the RTB or gov.ie guidance for the rules applying to your tenancy.