Irish Water has defended its response after it was found there were unsafe levels of chlorine in the water supply in Meath.
Customers in the Kilcloon area near Dunboyne are being told not to use the water for washing, drinking or preparing food.
Some people have developed rashes, while a number of children have become sick.
Locals have expressed concern over the time it took the company to issue its warning.
Head of operations at Irish Water, Kathleen Walsh, outlined when the firm was first notified of the problem.
“We received a notification at the start of the process on Tuesday evening, and there was one notification late on Monday night, so the whole process started, and, as I said, it culminated yesterday with the results of the test showing that we had elevated levels there,” she said.