A renowned barrister of his day, his work included representing Charles Haughey in the Arms Trial and to act for Gulf Oil in the Whiddy Island Disaster (1979) and for the owners of the Stardust fire venue (1981): he was the country's advocate of choice for two decades.
Judicial writing is often dry, but the advocate's panache occasionally surfaced: in McGarry v. Sligo County Council [1991] 1 Irish Reports 99, the case which prevented the Carrowmore megalithic tombs complex from being turned into a pithead, he quoted a Yeats verse and heartily endorsed a Swedish archaeologist's rhetorical question, 'Do the Irish have no pride?’ In Norris v. Attorney General [1984] I. R. 36, heard only a few months after McCarthy's appointment, his dissenting judgment is a tour de force of classic liberalism.
On 21 August 1992, just weeks before his sudden death, McCarthy delivered a coruscating dissent in Attorney General v Hamilton [1993] 2 I. R. 250, the case that upheld the Reynolds government's claim to absolute confidentiality for cabinet discussions.
This was in the context of the Hamilton tribunal enquiries into the issue of export credit insurance: McCarthy appended to his judgment the civil service note of what Reynolds had said the government had decided on that topic.
Though McCarthy was in the minority (with Mr Justice Séamus Egan), the absolute confidentiality found to attach was removed by the twelfth amendment to the constitution of 1997.