Interpretation Act
General Principles
Acts are public documents and judges must take judicial notice of them. They are not required to be proved in court. Statutory instruments must be formally proved.
The courts may make use of matter that accompanies the legislation. This allows reference to marginal and side notes in Acts but not statutory instruments. Courts may regard to the long title and short title together with the heading.
Acts are presumed to continue in force and apply at all times as the occasion requires. This is based on the argument that Acts cannot become redundant by non-application or non-use.
Function and Scope
Interpretation Acts have been in force since the end of the 19th century. They provide general rules on the interpretation of statutes. The latest interpretation act is the Interpretation Act 2005.
The purpose of the act is to give general direction to the courts on the interpretation of statutes. However, it a basic constitutional principle that the courts have exclusive power to interpret statutes. It is not a matter for the Oireachtas or Parliament.
The Interpretation Act applies to both acts and statutory instruments. The 2005 Act applies to all existing statutes.
This is a difference in the approach to that of the earlier interpretation legislation which applied to later legislation except in so far as a contrary intention appeared in the act itself or in the case of a statutory instrument made under it. The purported application to earlier legislation raises questions as to whether the Interpretation Act might dislodge the pre-existing interpretation of legislation.
Purpose / Intent where Obscure or Ambiguous
The Interpretation Act provides that in interpreting and construing provisions of an Act or those relating to penalties and sanctions that are obscure or ambiguous or that on a literal  interpretation would be absurd or would fail to reflect the plain intention of the act, the provision is to given a construction that reflects the plain intention of the Oireachtas or the Parliament where this intention can be ascertained from the Act as a whole.
To a large extent, the Interpretation Act embodies existing principles of statutory interpretation.The reference to obscurity is relatively new but may be similar in practice to ambiguity. Obscurity may simply refer to difficulty of interpretation.
See the separate chapters in relation to the approach of the courts to absurd or ambiguous interpretations. They are those which fail to give expression to the evident intention of the lawmakers.
As with the approach under existing common law, the plain intention in this context refers to the intention which might reasonably be inferred from the entire context of the Act. The above provision might be argued to detract from the approach of the Supreme Court in favouring a literal interpretation over a purposive interpretation on the basis of constitutional considerations.
Arguably the above provision gives a mandate to the courts to approach legislation from a purposive point of view. However, the courts’ functions remain as interpreters of the legislation.
The purposive approach is not applied to penal or criminal legislation. Criminal legislation should generally be clear and unambiguous. The lawmakers are not to be presumed to have criminalised conduct unintentionally.
Reflect Changed Circumstances
The Act  provides that in construing a provision of an act or statutory instrument the court may make allowances for changes in the law and  social conditions, technology, the meaning of words used in that act or statutory instruments and  other relevant matters which have occurred since the date of passing of the act or the making of the statutory instrument but only in so far as the text, purpose, and context permit. This provision reflects the existing principles of legislation set out separately.
Interpretation of Particular Words and Phrases I
The Interpretation Act provides certain important general rules of interpretation of particular words. They apply in the absence of an indication to the contrary.
Words implying the single are also to be read as including the plural and vice versa.
The presumed reference to the plural and references to the singular can be significant in some contexts. The question arises as whether there is a contrary intention. When reading a word as a plural makes a significant or radical change, the courts will impute an intention to the contrary.
References to one gender is to be implied and imports a reference to the other gender. A reference to the feminine gender is to be read as including the masculine gender.
A person is presumed to include a body corporate, company, or a non-corporated body or person.
Interpretation of Particular Words and Phrases I
References to a child in legislation after 1976 includes an adopted child. Reference in legislation after mid-1987 to a child is presumed to include a non-marital child.
A reference to the distance between two points and every reference or distance from a point to another is presumed to be by way of a straight line on the horizontal plane.
In the case of references to periods of time which were expressed to begin on, or are reckoned from a particular day, the day is deemed to be included in the period and a period of time is expressed to end on or to be reckoned to a particular day. The day should be deemed to be included in the period.
The Interpretation Act provides that the courts may where the context otherwise requires, deviate from the definition where the context requires that it should not apply.
This Interpretation Act defines a range of commonly used words and statutes. The interpretation is to apply in the absence of provision to the contrary.
Land is presumed to include buildings, houses, land covered by water and rights or estate in land. There are definitions of some commonly used words and expressions.