The Water Advisory Body, set up to advise on measures to improve Irish Water, has said it is concerned at the number of long-term boil water notices being issued.
In a quarterly report, the agency also noted concerns about the operating standards of Irish Water's consumer support systems.
Twenty-one areas were under a boil water notice in the second quarter of this year and 20 of these areas remained on the notice for more than 30 days.
The Water Advisory Body described that as most concerning.
It also shows Irish Water's five-day response rate for complaints was also the lowest since reporting began against this metric in 2018.
The report notes, however, that there has been an increase in customer satisfaction rates and also a drop in the number of priority areas where water treatment needs to improve.
In a statement, Irish Water said it recognised the scale of the challenge in improving water infrastructure, but said that progress is being made.
A senior inspector at the Water Advisory Body has said significant investment is required by Irish Water to address the issues of water quality.
Michelle Minihan said that at the end of Q1 to March 2022, there were just over 13,500 people on boil water notices.
She told RTÉ's Morning Ireland that the vast majority of those notices were long term and had been in place for more than 30 days, meaning there would have to be "significant investment" to address the issues.
Ms Minihan said that today alone, there are 22 boil water notices in place, serving about 5,500 people, while 18 of those notices are long term.
"It is a huge frustration and burden for communities that are affected by it," she said.
Ms Minihan said the board's report highlights that Irish Water’s customer response times are at the at the lowest level they have been at since this metric was set up to measure the utility company’s performance.
She added that their incident closeout, which is required to be done within two months, is also at its lowest level.
"There's clearly a need for Irish Water to improve their customer complaints management, which we've highlighted in our quarterly report, but also a need to address these issues for communities affected by boil water notices.
However, the report does show some positive points, she said, such as wastewater treatment plants down to 92 from 148 and areas like Cork city and Shannon in Co Clare now having new treatment plans that meet the directive standards.
However, she said it is balanced out by concerns about having to readd in other areas, such as Kinsale and Clonakilty in Co Cork to priority areas, because the treatment plants that are there are not now able to meet compliance with the Urban Wastewater Directive.