The transmission systems in Ireland and Northern Ireland consist of the high voltage networks for the transmission of bulk electricity. In Ireland, EirGrid is licensed by the CER to act as Transmission System Operator (TSO) and is responsible for operating the transmission system and for carrying out its maintenance and construction. Likewise, in Northern Ireland SONI is licensed to act as TSO, with NIE licensed as the owner of the system.

The Regulatory Authorities work on all-island SEM transmission issues to the benefit of both jurisdictions. This includes areas such as harmonised transmission investment and outage planning, transmission loss adjustment factors (TLAFs), Grid Code requirements, transmission tariffs, and ancillary services and related payments.