Explainer
Ireland and Northern Ireland share one wholesale electricity market but run two retail markets, with different regulators, network operators and market systems. Here is who does what, how switching and settlement work, and what is changing.
By Christopher Fagan, Managing Partner, PEF Consulting Partners
The Single Electricity Market, or SEM, has traded wholesale electricity across Ireland and Northern Ireland since 1 November 2007. New market arrangements took effect on 1 October 2018, built around day ahead, intraday and balancing markets alongside a capacity mechanism. SEMO, a joint venture between EirGrid and SONI, operates the market, and the SEM Committee, which brings together the CRU and the Utility Regulator, governs it. EirGrid runs the transmission system in Ireland and SONI in Northern Ireland.
Retail supply is regulated separately, by the Commission for Regulation of Utilities in Ireland and the Utility Regulator in Northern Ireland, and each market has its own network operator and central systems.
Suppliers exchange standard market messages with the central systems for registrations, changes of supplier, meter readings and consumption data. In Ireland these messages flow through a central messaging hub, and each supplier runs its own messaging software, known as EMMA, to connect to it. Every message type, format and timing rule is set out in the market design, so a supplier’s systems have to follow it exactly, and any change to the design means change for every supplier.
A customer switches by agreeing a contract with a new supplier, which then registers the MPRN through the market systems. A meter reading at the switch closes the old account and opens the new one, and the customer keeps the same meter and MPRN. See what is an MPRN.
The SEM settles imbalances over half hour periods, with prices calculated every five minutes and averaged across each half hour, and the day ahead market has priced in half hour periods since March 2025. Behind that, ESB Networks aggregates meter data for each supplier so that what suppliers bought can be matched with what their customers used.
The full list, with dates, is in what energy suppliers must change before 2030, and our Ireland page sets out how we help suppliers through it.
The all island wholesale electricity market for Ireland and Northern Ireland, in place since November 2007 and reformed in October 2018.
SEMO, a joint venture between EirGrid and SONI. The SEM Committee, which brings together the CRU and the Utility Regulator, governs it.
The Commission for Regulation of Utilities (CRU) in Ireland and the Utility Regulator in Northern Ireland.
The Retail Market Design Service, ESB Networks’ service that maintains the design and documentation of Ireland’s retail electricity market.
Meter Registration System Operator: the ESB Networks function that records which supplier serves each MPRN, runs changes of supplier and aggregates meter data for settlement.
Related reading: what is an MPRN, smart meters in Ireland, Ireland’s 2030 energy targets.
If you are entering the Irish market, changing platform or preparing for the next market design change, we can help you get the market interfaces, data and processes right first time.