The Eirgrid interconnector is a 500MW under sea link running between Deeside in north Wales and Woodland, County Meath in Ireland.
The East-West interconnector, which has taken three years to complete and cost €600million, was praised by Secretary of State Ed Davey as an “impressive feat of engineering”.
He added that it will “improve our mutual energy security” and will give the UK access to Ireland’s “abundant wind energy”.
The European Investment Bank provided a €300 million loan for the project.