You can apply to the Residential Tenancies Board (RTB) for dispute resolution, usually starting with free mediation where an agreed settlement may be recorded. If mediation does not resolve the issue, an adjudicator can consider written evidence and submissions, and either party may usually appeal to a Tenancy Tribunal. The RTB then issues a legally binding determination order, which can require payment or other action, although the outcome depends on the facts and evidence in the case. Check the RTB’s current procedures and deadlines, and consider professional legal advice for a complex dispute.