The law of Ireland consists of constitutional, statutory, and common law. The highest law in the State is the Constitution of Ireland, from which all other law derives its authority. The Republic has a common-law legal system with a written constitution that provides for a parliamentary democracy based on the British parliamentary system, albeit with a popularly elected president, a separation of powers, a developed system of constitutional rights and judicial review of primary legislation.

The sources of law in both the Republic of Ireland and Northern Ireland reflect Irish history and the various parliaments whose law affected the island of Ireland down through the ages.

The Brehon Laws were a relatively sophisticated early Irish legal system, the practice of which was only finally wiped out during the Cromwellian conquest of Ireland (ending in 1653). The Brehon laws were a civil legal system only – there was no criminal law. Acts that would today be considered criminal were then dealt with in a similar manner to tort law today. A perpetrator would have to compensate the victim, rather than having a punishment, such as imprisonment, imposed upon him or her.

The Parliament of Ireland made laws for the Lordship of Ireland and Kingdom of Ireland from 1297 until the end of 1800.

After the kingdoms of Ireland and Great Britain were united in 1800, the Parliament of the United Kingdom made laws for Ireland. This continued in the south until 1922.

The first (1919-21) and second Dáil (1921-22) sat in opposition to British rule in Ireland. The laws passed by the first and second Dáil had no official legal effect.

The Irish Constitution was enacted by a popular plebiscite held on 1 July 1937, and came into force on 29 December of the same year. The Constitution is the cornerstone of the Irish legal system and is held to be the source of power exercised by the legislative, judicial and executive branches of government. The Irish Supreme Court and High Court exercise judicial review over all legislation and may strike down laws if they are inconsistent with the constitution.

The Constitution can be amended only by referendum. A proposal to amend the Constitution is introduced into Dáil Éireann (the lower house of parliament) as a bill and if passed by the Dáil, and passed or deemed to have been passed by the Senate (the upper house), is put to the people. Only Irish citizens resident in the state may vote. There is no threshold for such referendums and a simple majority of voters is sufficient for a proposal to be passed. Once passed by the people, the President signs the referendum bill into law. As of November 2011, there have been 33 such referendums: 23 of which were approved by the people and 10 of which were rejected. The constitution was also amended twice during an initial transitional period of three years following the election of the first President of Ireland when amendments could be made without recourse to the people.