Career
He later came to prominence as a lobbyist and publicist for the tobacco industry. From 1973 to 1977, O'Mahony was a policy advisor to the Tánaiste and Minister for Health, Labour Party leader Brendan Corish. When Labour returned to opposition after 1977 general election he continued as an advisor to Corish's successor, Frank Cluskey.
O'Mahony had himself stood unsuccessfully as a Labour candidate at the 1969 and 1973 general election in the Dún Laoghaire and Rathdown constituency. However, in 1981 he was elected on the Administrative Panel to the 15th Seanad, and was re-elected twice, serving until the dissolution of the 17th Seanad in 1987. After leaving politics, O'Mahony became a public affairs consultant in 1989.
He is also an Associate Lecturer in European Studies at the Institute of Public Administration in Dublin. O'Mahony, later became known as the "public face" of the Irish Tobacco Manufacturers Advisory Committee (ITMAC), of which he was director and which shared an office in Dublin with O'Mahony's company CIPA, and in 1992 O'Mahony's name was recorded as the donor of IR£3,000 donated to the Progressive Democrats on behalf of ITMAC. As a lobbyist against plans for legislation to protect workers against passive smoking, O'Mahony was named in 1999 as having been involved in lobbying by ITMAC which Dr Fenton Howell, vice-president of the Irish Medical Organisation, claimed "secretly manipulated and misled a group advising the minister for health on new smoking regulations". After hearing O'Mahony's evidence, the chairman Batt O'Keeffe, told Mahony that some of the points made about his conduct were "well-founded", and recommended that "in future deliberations he would be conscious of the public interest and people's health".
In 2001, Howell told a sub-committee that O'Mahony had been "less than candid in his replies" to the committee. O'Mahony was one of three former senior officials of the Irish Labour Party reported to have had ties with the tobacco industry.