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.

About companies in Ireland

Irish companies fall into three categories:

• Private limited companies, which carry the suffix "Limited" (Ltd) or "Teoranta" (Teo), and whose shares are privately held.
• Public limited companies, which carry the suffix "plc" or "cpt" and whose shares may be listed on a stock exchange. Where this is the case, it is noted in this article.
• Company limited by guarantee, this type of company has members, not shareholders, as such generally limited to trade associations and not-for-profit bodies.

Companies in the Republic of Ireland must be registered with the Companies Registration Office, and comply with the provisions of the Companies Acts. Companies in Northern Ireland must be registered with the Department for the Economy's Companies Registry. Certain Northern Ireland organisations are dealt with on a UK-wide basis and are dealt with by Companies House.

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 mutual bodies, such as building societies, are also included on this list.

Some entries such as Three 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 Ireland employs over 5000 people.

Largest firms

This list shows firms in the Fortune Global 500, which ranks firms by total revenues reported before 31 March 2017.^[1] Only the top five firms (if available) are included as a sample.

Rank	Image	Name	2016 revenues (US$M)	Employees	Notes
272		Johnson Controls	37,674	209,000	Multinational industrial conglomerate manufacturing electronics for the automotive industry and commercial electrical equipment. After a full merger with Tyco International in 2016, the firm established headquarters in Cork, Ireland.
305		Accenture	34,798	384,000	Global professional services and management consulting firm. The company formed as a spin-off from the U.S.-based Arthur Andersen in 1989 and moved their headquarters to Dublin in 2009. Accenture's clients include more than three-quarters of the Fortune Global 500.
363		CRH plc	29,973	86,778	Diversified building materials and construction holding company. The firm provides heavyside products and materials (cement, aggregates and concrete), lightside products (glass, fencing), and retail product distribution (merchant chains).
377		Medtronic	28,833	98,017	Multinational medical device design, development and manufacturing. company. After acquiring Dublin-based Covidien in 2014, the firm moved their headquarters from the United States to Ireland while retaining operational headquarters in Fridley, Minnesota.

Notable firms

This list includes notable companies with primary headquarters located in the country. The industry and sector follow the Industry Classification Benchmark framework. Organizations which have ceased operations are included and noted as defunct.