Irish Water has confirmed that boil water notices affecting 11,300 customers in Co Roscommon have been lifted following inspections by the Environmental Protection Agency.
Work on water plants in Killeglan and Castlerea was completed early this year, and following testing by the EPA and Health Service Executive the notice has been lifted.
Residents in the area had been boiling their drinking water since 2009.
A separate notice affecting 6,000 people in the Boyle area was lifted in May of this year.
However, 5,400 customers in the northeast of Roscommon remain on boil water notices since March 2014, with the company saying it is hopeful this will change by late 2015.
Speaking on RTÉ's News at One, Gerry Grant, Head of Asset Management at Irish Water, said the removal of boil water notices in the Roscommon area is a "long-term challenge".
He said customers still affected by the notice, mostly in the Strokestown area, should see it lifted by the end of November.
Mr Grant added that customers in Williamstown, Co Galway, should also see a boil water notice in the area lifted by the end of the year.
Welcoming the news, he said "this is probably the lowest number of people on boil water notices for decades in this country".
Minister for the Environment Alan Kelly said he had no doubt similar successes will be replicated throughout the country.
Analysis: George Lee, Agriculture and Environment Correspondent
Over 90% of all the people in Ireland on boil water notices at the start of 2014 lived in Roscommon.
The county has 21 public water supplies serving its 48,800 people. However, 43% of those people have been under instructions to boil their AP (Aqua-Pure) water before drinking it because of contamination with a microorganism called Cryptosporidium.
The nature of eroded limestone rock close to the land surface throughout the county means groundwater in the area is heavily influenced by surface water runoff.
This makes the water supply in the region particularly vulnerable to Cryptosporidium contamination.
It is only in recent years that our health and environmental authorities have demanded that drinking water must be monitored for contamination by Cryptosporidium.
In all boil water notices were issued for eight of the public water supplies in Roscommon due to Cryptosporidium.
In addition the Environmental Protection Agency ordered remedial work on three further sites which it deemed to be at risk from Cryptosporidium over the past ten months.
Today's announcement means that four of the eight boil water notices that were in place at the start of this year have now been lifted.
The first two, at Boyle and the Boyle/Ardcarne, affecting 6,000 people, were lifted two months ago following the installation of new water facilities.
A further 11,300 people will benefit from the lifting of the further two boil water notices at Killeglan and Castlerea today.
Altogether it means that 81% of Co Roscommon residents affected by boil water notices at the start of this year have now been given the all-clear to drink their own tap water.
Remedial work towards the removal of the remaining boil waters notices is ongoing.
Irish Water expects that a temporary filtration and UV treatment system will be in place late this year that will allow the remaining Roscommon boil water notices to be lifted.