Costs for the upgrade of the country's largest wastewater treatment plant increased by almost 25pc over three years.
The Ringsend treatment plant in Dublin, which treats 40pc of the entire country's sewage, is expected to need around €500m to boost its overloaded capacity.
It had previously been costed at around €400m but the Commission for the Regulation of Utilities (CRU) reported the forecast expenditure had increased by €89m.
The CRU's latest monitoring report on Irish Water's capital expenditure programme accepted that costings could change over time.
"The CRU recognises that as the projects pass through various stages of project development, for example where planning decisions require refinements to the scope of a project, the cost forecasts may be refined accordingly. These projects tend to be progressed and costed in phases," it said.
Irish Water said work on the structure of the plant began in 2018 but the nature of the installations had not been finalised then. In 2019, An Bord Pleanála granted permission for additional works.
"Through the planning process, the forecasts have been reviewed and updated," Irish Water said. "For all the works at Ringsend wastewater treatment plant, investment approval is sought for each project phase through the Irish Water Board in line with the public spending code."
The CRU examined just under €2bn worth of capital spending by Irish Water from 2017-2019 and said there was evidence of "optimism bias" as the utility had not managed to achieve all it set out to do.
"There is evidence that Irish Water's under-delivery in the period has resulted in failures to reduce the risks to water supply quality," the report said.
It acknowledged Irish Water "faced challenges in terms of data and understanding its asset base" when developing its investment plan.
"The CRU has required that Irish Water carries out an external review of its approach to capital investment planning including its approach to costing and prioritising its projects and programmes," it said.