[7] As auditor of the CHS, his inaugural address was on the subject of "Republicanism Reviewed" and featured contributions from Conor Cruise O'Brien, John A. Murphy, Bernadette Devlin McAliskey, Mairéad Corrigan, Noël Browne and John Brooke, 2nd Viscount Brookeborough.
[4] He represented Christy Burke in a defamation action in 1996[15] and Eircom in a case related to the publication of a phone sex line in their telephone directory.
[16] He defended Ritchie Neville and Jason "J" Brown of the boyband 5ive following charges arising out of an altercation in Temple Bar, Dublin in 2001.
[17] He worked as senior counsel for the Morris Tribunal into allegations of corruption in the Donegal division of the Garda Síochána.
[28] He developed the substance of Irish common law defences of duress and provocation in the Gleeson and McNamara cases respectively, in both instances deploying tests using mixed standards of objective and subjective elements.
[35] In his concluding remarks on his findings, he criticised the Gardaí and the Child and Family Agency for not having organisational mentalities of learning from their errors.
[36] Drew Harris, O'Sullivan's successor as commissioner, said on its publication that the report was "difficult reading for the organisation" and the Gardaí would move to have "an open and inclusive culture".
He suggested that instead of being run in a format akin to criminal trials, the main aspect of the inquiry should be directed from the tribunal itself.
[11] He was a founder-member of the National Symphony Chorus RTÉ Philharmonic Choir and a member of the board of the Irish Baroque Orchestra.