The €1bn Celtic Interconnector between Cork and France was due to begin operating in early 2027.
It would allow Ireland to import power to boost the country’s strained electricity supply, serving up to 450,000 households at full capacity if needed.
However, Eirgrid and its French partners, Reseau de Transport d’Electricite (RTE), said there has been a delay to the manufacture and installation of the undersea cables.
“Adjustments have been required to the sub-sea marine cable programme, which has resulted in an adapted burial programme, extending the time required,” Eirgrid said.
The project has been under construction since 2023
“As a result, the project’s commissioning date is now expected for spring 2028, rather than 2027.”
It warned that the timeline could be stretched further, saying: “Adverse weather conditions could impact this date, if marine cable burial cannot be undertaken.
“However, the programme is scheduled for spring and summer to take advantage of more optimal conditions.”
The flagship energy project has been in the planning for over a decade and under construction since 2023.
It received more than €500m in EU funding, while the rest came from Eirgrid and RTE.
The landing point is near Youghal, Co Cork
Further unspecified costs will be incurred now because of the delays, but Eirgrid said the EU would be asked to extend its grant aid to help cover them.
Significant works on the 575km interconnector have been carried out on land in both countries.
The landing point in Ireland is near Youghal, Co Cork, with the high voltage cables running inland to a converter station and substation at Ballyadam and Knockraha between Midleton and Cork city.
Works are due to finish there next year, while similar works at the French landing point in Brittany are also well under way.
The 500km undersea element of the project is the more complex undertaking, and it is this that has run into difficulties.
It is a setback from an energy-security and climate-action perspective
Eirgrid said work had otherwise been on schedule and a hold-up was not unexpected for a project of this nature.
“It is not unusual for timelines to evolve on complex cross-border infrastructure projects of this scale, particularly where marine works are involved,” it said.
However, it is a setback from an energy- security and climate-action perspective.
Ireland has begun importing significant amounts of electricity via two interconnectors in the UK, which has reduced the need to burn gas to generate power domestically and has enabled national greenhouse gas emissions to fall.
Increased interconnection is also promoted as a way to boost electricity supply to meet the country’s fast-growing demand until offshore wind projects get under way.
In the longer term, it is proposed that Ireland could benefit from interconnection with the wider European market via France, with potential to export surplus electricity when the offshore wind sector takes off.
The Celtic Interconnector will have the capacity to carry 700 megawatts of electricity, roughly equivalent to the output of a large offshore windfarm.