Just more than 20% of households have still not registered their septic tanks and waste water systems, before the deadline expired last Friday.
A statement issued by Environment Minister Phil Hogan thanked the 400,000 households who have registered, and appealed to those who have not signed up to do so.
He also confirmed that inspections will begin this summer and that householders who have not registered will be liable for a fine of up to €5,000.