Mould is a symptom. The question a specialist asks first is whether the plaster and timber behind it are still wet, and they answer it with a meter. Painting or bleaching a patch on a wall that still reads damp only hides it for a few weeks. Thick masonry in the city centre, painted brick in Blackrock and stone cottages toward the harbour hold moisture far longer than a timber-frame estate house in Carrigaline. Basements and rooms below garden level near the river stay humid after the floor looks dry. Wardrobes and sofas pushed against an outside wall hide the first growth. Tell the specialist how long the room has been damp, whether anyone in the house has asthma or a chest condition, and what has been sprayed on it so far. Removal of badly affected plaster, containment and filtered air are their decisions, priced by them after they see it.
Meter readings first. If the wall is still wet, the mould work waits for drying, or the firm handles both in order.
Pull furniture away from affected walls, photograph the patches with a date and stop using household bleach on porous plaster.
Salt air in Cobh and Passage West slows drying on exposed gable walls. Harbour-side houses often need longer with the equipment running.