For most private tenancies in Dublin, a landlord cannot increase the rent more than once in 12 months and must follow the applicable legal cap, generally the lower of 2% or inflation. They must give proper written notice, usually at least 90 days before the new rent takes effect, and provide required information about the calculation and comparable rents. Some exemptions and newer tenancy rules may apply, so check the current guidance on the RTB or gov.ie. If you believe an increase is invalid, the RTB’s dispute resolution process can determine the issue based on your tenancy’s facts.