85% of septic tanks inspected in Co Mayo last year did not meet environmental standards.
Tests on septic tanks were carried out by local authorities across the country, on behalf of the Environmental Protection Agency.
Nationally, there was a 51% failure rate, and 26% were found to be a risk to human health or the environment.
A lack of maintenance and desludging was identified as a key issue in systems failing inspection.
In Mayo, 67 inspections were carried out last year, with an 85% failure rate.
Since 2013, 300 septic tanks in Mayo failed inspections, of which only 60% have been fixed.
In Galway, Roscommon and Leitrim, less than half of systems that failed have been fixed.
The EPA is warning that if people do not maintain their septic tank, it can contaminate their own or a neighbour's well, putting human health at risk.