This is a list of notable companies based in Ireland, or subsidiaries according to their sector. It includes companies from the entire island. The state of the Republic of Ireland covers five-sixths of the island, with Northern Ireland, part of the United Kingdom, covering the remainder in the north-east. Each has separate regulatory and registration authorities.
Some entries on this list may be statutory corporations - a business form governed by a board appointed by an Irish Government minister under a particular piece of legislation - not formally companies as such, but are effectively treated as such and are included on this list as they are usually large business. For more details on this form of governance, see state-sponsored bodies of Ireland.
Some entries such as O2 Ireland and Intel Ireland are subsidiaries of foreign companies. These subsidiaries are listed because the Irish economy is significantly dependent on foreign companies in comparison to other nations and these multinational companies are major Irish employers and tax contributors - Intel employs over 5000 people.