There will be an extra seat for the Dublin and South constituencies at the next European elections, a boundary review published on Monday states.
Counties Laois and Offaly will move into the South constituency from the Midlands-North-West, which remains a four-seater, under the changes.
The review followed the reallocation of the UK's 73 European Parliament seats, which have been proportionately divided among the remaining member states. Ireland received two additional seats, bringing the total to 13.
Britain and Northern Ireland will cease to have any MEPs after the end of March next year when the UK leaves the EU.
The next European Parliament elections are due to take place at the end of May.
The review was carried out by a committee which included Mr Justice Robert Haughton of the High Court; Peter Finnegan, clerk of Dáil Éireann; Martin Groves, clerk of the Seanad; John McCarthy, secretary general of the Department of Local Government, and Peter Tyndall, the Ombudsman.
Its results mean that Dublin will become a four-seat constituency, while Ireland South – extending from Wicklow to Kerry and from Waterford to Offaly – will become a five-seater.