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How International Law Works

International treaties, custom and state practice and judicial decisions are important sources of international law.

  • Overview
  • Treaties
  • Custom and State Practice
  • Judicial decisions


What is International Law?

International law is the term given to the rules which govern relations between states.

Despite the absence of any superior authority to enforce such rules, international law is considered by states as binding upon them, and it is this fact which gives these rules the status of law. So, for example, where a state wishes to avoid a particular rule, it will not argue that international law does not exist, but merely that states have not agreed that such a rule is to be binding upon them, or that the rule does not apply to the particular circumstances.

Unlike national or domestic law, international law is not set down in any legislation approved by a parliament. Even multilateral treaties do not apply to all states, but only to those which have consented to be so bound, by signing and ratifying or acceding to them. As a result, the precise rules of international law are often more difficult to identify than national laws, and may be found in a variety of sources.

Article 38 of the Statute of the International Court of Justice directs the Court to apply the following sources of law in deciding disputes:

"(a) international conventions, whether general or particular, establishing rules  expressly recognised by contesting states;

(b)  international custom, as evidence of a general practice accepted as law;

(c)  the general principles of law recognised by civilised nations;

(d) … judicial decisions and the teachings of the most highly qualified publicists of the various nations, as subsidiary means for the determination of rules of law.”



Whereas bilateral treaties or treaties between only a few states may best resemble a contract between individuals, multilateral treaties such as the UN Convention on the Law of the Sea are often referred to as “law-making treaties” in that they represent, as close as the international legal order gets, to international legislation. Widely accepted multilateral treaties, such as the Vienna Convention on the Law of Treaties itself, are often taken as firm indicators of the content of customary international law on the topic.  

The Irish Treaty Series contains further information on treaties and a database of treaties to which Ireland is a party.

The United Nations Treaty Series contains further information on treaties and a database of treaties. The United Nations has also published a Treaty Handbook providing information on many aspects of treaty law and practice and includes a useful glossary of many of the terms used when referring to treaties, for example ratification, accession, declaration, etc. 



Custom and State Practice

Custom and State Practice

In the absence of a treaty governing relations between two or more states on a particular topic, what is important is evidence of the existence of consensus among states as to what the law should be, or, in other words, state practice combined with recognition that a certain practice is obligatory. If sufficiently widespread and consistent, such practice and consensus may constitute customary international law.

Evidence of custom may be found among the following sources:

  • diplomatic correspondence,
  • opinions of official legal advisers,
  • statements by governments,
  • United Nations General Assembly resolutions,
  • comments by governments on drafts produced by the International Law Commission,
  • the decisions of national and international courts

However, as with all matters of evidence, the weight which can be given to a particular statement varies greatly depending on the circumstances in which it was made.

Judicial decisions

Judicial Decisions

Article 38 of the Statute of the International Court of Justice includes among the possible sources of international law, “judicial decisions” as a subsidiary means for the determination of rules of law.

Apart from decisions of international judicial bodies, decisions of a national court may amount to a statement of what that court considers to be international law on a particular matter. Such a decision would only carry weight as evidence of international law where the court is of very high standing and where the international law issue is central to the case and receives careful consideration. So, for example, important decisions of the United States Supreme Court (such as 1900 case, The Paquete Habana), the House of Lords (such as the Pinochet Case) and the Irish Supreme Court (such as The Government of Canada v The Employment Appeals Tribunal) have influenced the development or interpretation of international law.

In addition, decisions of national courts may in themselves be evidence of state practice on a particular topic. In an important early case, The Lotus (1927) the Permanent Court of International Justice examined decisions of French national courts in order to discover what the state practice of France was on the subject at hand.  

The following is a list of some Irish cases which consider international law issues:

Interaction between Irish law and International Law

• In re Ó Láighléis [1960] IR 93
• State (Gilliand) v Governor of Mountjoy Prison [1987] ILRM 278
• ACT Shipping (Pte) Ltd v Minister for the Marine and Others [1995] 3 IR 407
• Horgan v An Taoiseach, Minister for Foreign Affairs [2003] 2 IR 468
• Crotty v An Taoiseach [1987] IR 173

State Immunity

• Zarine v Owners of SS Ramava [1942] IR 148
• Saorstát and Continental Steamship Co v De Los Morenas [1945] IR 291
• State (Summers Jennings ) v Furlong [1966] IR 183
• Government of Canada v Employment Appeals Tribunal [1992] 2 IR 484
• Fusco v O'Dea [1994] 2 IR 93
• The Marshall Gelovani [1995] 1 IR 159
• McElhinney v Williams [1995] 3 IR 382
• ACT Shipping (Pte) Ltd v Minister for the Marine [1995] 3 IR 406
• Schmidt v Home Secretary of the Government of the United Kingdom [1997] 2 IR 121


For Further Information on

Ireland’s stance on certain international legal issues, read our statements on International Law Issues