Water sources serving more than 70,000 people are well below normal levels, prompting fears of supply shortages next summer.
The Environmental Protection Agency (EPA) said a river in Limerick and three lakes in the midlands have not yet returned to "normal" levels after the summer drought, which is "of concern".
While January, April, November and December were wetter than normal, Met Éireann said 2018 was drier than average. Casement Aerodrome in Dublin saw its driest year since 2005.
The lack of rainfall means water levels have not recovered in some areas following the drought, which could lead to problems later in the year.
Irish Water extracts water from more than 1,000 individual sources, including 280 surface water sources (rivers and lakes) which provide 80pc of total supply. The remainder are groundwater sources, including springs and boreholes.
The EPA said flows in the River Deel in Limerick have been "very low" since August. This provides just over 2.6 million litres a day to the Abbeyfeale supply, which serves a population of 6,600.
Levels at Lough Lene in Westmeath are "very low", with concerns also for Lough Owel and Lough Bane in Westmeath and Meath, where the lowest levels in over 30 years were recorded in 2018.
Lough Owel serves Mullingar and produces 18.5 million litres of treated water a day to a population of 49,000. The Bellany treatment plant abstracts 4.7 million litres a day from Lough Lene in Westmeath, and serves Castlepollard and a total population of 7,700.
Both Lough Bane and Clavin's Bridge serve the towns of Kells and Oldcastle in Meath, and a total population of almost 11,000. The Lough Bane water treatment plant produces almost 2.5 million litres a day, and Clavin's Bridge another 1.1 million.
"However, the plants serve the same network so any impact on one could affect the entire population of 11,000," Irish Water said.
Below-average rainfall over the coming months and a dry summer could result in problems, the EPA warned.
"Of concern is that water levels in these lakes have not recovered much since the summer; Lough Owel and Lough Bane are well below their long-term average for December and...are well below the water levels encountered in December 2017. The net effect of this may be seen during the summer of 2019 if we get another dry summer, unless there is a significant amount of rain in the intervening period," the utility said.
Irish Water will this year produce a National Water Resources Plan, which will analyse the availability of drinking water sources and map out how to meet future demand over the next 25 years. It is likely that some sources will be abandoned because they are not sustainable.
Head of asset management Sean Laffey said: "The 2018 drought conditions demonstrated the vulnerability of many of our water supplies. This drought brought into sharp focus the size and scale of the challenge facing Irish Water to provide for the current and future water needs for homes and businesses, whilst ensuring in the long term (that) water sources are environmentally sustainable."