Estate agents and letting agents will often have their own complaints process. Many agents belong to a professional redress scheme which may be able to sort out problems if you're unhappy with the service you've been given by your agent.
Many problems can be resolved by talking to someone at the Housing Executive. When this doesn't sort out the problem you can use the Housing Executive's formal complaints procedure. There are two stages to the complaints procedure. If you're not happy with the outcome of the formal complaint you might be able to ask the Ombudsman to look at your complaint or even take a Judicial Review against the housing association.