The maxim expressio unis est exclusio alterius ("to express one thing is to exclude another") allows the courts to imply that where an Act applies a rule to a particular situation, the Oireachtas intended to confine the rule to that situation, and not to apply it in any wider context.
The maxim was applied in Kiely v Minister for Social Welfare and again in O'Connell v An t-Ard Chlaraitheoir . It was succinctly illustrated by Henchy J in Keily when he said:
"The fact that article 11 (5) allows a written statement to be received in evidence in the specified limited circumstances means that it cannot be received in other circumstances: expressio unius est exclusio alterius ".
The maxim has limitations, however, and will not apply where a legislative provision merely states a particular aspect or a particular application of a more general rule of law. This was accepted by the Irish courts in The State (Minister for Lands and Fisheries) v Judge Sealy and in Inspector of Taxes v Arida Ltd .