[2] His first major decision was in McGee v. The Attorney General which invalidated a law prohibiting the sale of contraceptives in Ireland.
[1] In 1983, he issued a dissent in Norris v. Attorney General where the majority upheld a criminal ban on homosexuality.
[1] He said that the relevant parts of the legislation were unconstitutional "on the ground that by their overreach and lack of precision and of due discrimination, they trench on an area of personal intimacy and seclusion which requires to be treated as inviolate".
[18] He wrote a concurring opinion with the then Chief Justice Tom O'Higgins in 1980 in the case of Cahill v. Sutton which established the rule of standing in Irish constitutional law where Hench wrote: The primary rule as to standing in constitutional matters is that the person challenging the constitutionality of the statute, or some other person for whom he is deemed by the court to be entitled to speak, must be able to assert that, because of the alleged unconstitutionality, his or that other person’s interests have been adversely affected, or stand in real or imminent danger of being adversely affected by the operation of the statute.
[19]He also contributed to decisions establishing the right to legal aid in criminal trials and in the case of Crotty v. An Taoiseach which established the need for a referendum to incorporate new European Union treaties into Irish law where he wrote: There is, of course, nothing in the Constitution to prevent the Government, or any person or group or institution, from advocating or campaigning for or otherwise working for a change in the Constitution.
[6][17]Henchy also laid the grounds in favour of the harmonious judicial interpretation of the Irish Constitution as opposed to the literal approach following his judgment in the case Tormey v. Ireland given that the harmonious approach goes further, ensuring that the Constitution is internally consistent and not contradictory.
The harmonious approach was summed in Tormey thus: As indicated earlier in this judgment, Article 34, s 3, sub-s 1, despite its unqualified and unambiguous terms, cannot be given an entirely literal construction.
A judicial attitude of strict construction should be avoided when it would allow the imperfection or inadequacy of the words used to defeat or pervert any of the fundamental purposes of the Constitution.
The true purpose and range of a Constitution would not be achieved if it were treated as no more than the sum of its parts.Judge Henchy was also instrumental in shaping the jurisprudence surrounding the principle of proportionality as a core facet in Irish Constitutional law most notably in the case of Heany v. Ireland.
In his judgment on the case, he laid down the criteria to which courts should apply it stating: In considering whether a restriction on the exercise of rights is permitted by the Irish Constitution, the courts in this country and elsewhere have found it helpful to apply the test of proportionality, a test which contains the notions of minimal restraint on the exercise of protected rights and of the exigencies of the common good in a democratic society.
This is a test frequently adopted by the European Court of Human Rights (see, for example Times Newspapers Ltd. v. United Kingdom, (1979) 2 E.H.R.R.
The objectives of the impugned provision must be of sufficient importance to warrant overriding a constitutionally protected right.
[11] Henchy was awarded an honorary doctorate by Trinity College Dublin in 1990 and another by NUI Galway in June 1999.
[15] In July 2020, Chief Justice Frank Clarke writing for the Supreme Court in Friends of the Irish Environment v. The Government of Ireland said that he "fully agreed with the observations" of Henchy in approaching unenumerated rights in McGee and Norris.