An ESB subsidiary company and the construction firm Ascon have been convicted at Gort District Court of polluting a river in Derrybrien, in Co Galway, following a major bog slide there last year.
ESBI Engineering LTD and Ascon were prosecuted by Galway County Council for allowing polluted materials to enter a river for a period in October 2003.
The court said it would impose fines and make a decision on costs tomorrow.
The bog slide took place as a €60m windfarm was being built on a mountain over looking Derrybrien.
Tomorrow the court will hear a separate case which is being taken by the Shannon Regional Fisheries board against the companies.