Declan Kelly has resigned as chief executive and chairman of strategy company Teneo after allegations of drunken misconduct at a fundraising concert.
Mr Kelly had earlier lost his board seat at the campaign group Global Citizen and ceded some of his responsibilities as head of Teneo after the allegations.
The 1,250-person public relations firm has 33 offices around the world and describes itself as "the world's leading CEO advisory firm".
In a brief statement, the company said that its co-founder and chief operating officer Paul Keary has been appointed CEO, effective immediately.
In a statement, Teneo thanked Mr Kelly for his "leadership and dedication over the past ten years in building Teneo into the world’s preeminent CEO advisory firm".
Mr Kelly was accused of behaving inappropriately at a party in May linked to the high-profile concert put on by Global Citizen.
"On May 2nd I made an inadvertent, public and embarrassing mistake for which I took full responsibility and apologized to those directly affected, as well as my colleagues and client," said Mr Kelly in a statement posted on the company's website.
"A campaign against the reputation of our firm has followed and may even continue in the coming days. However, regardless of the veracity of any such matters I do not want them to be an ongoing distraction to the running of our company."
He added that he had taken the decision to step down in order to protect the employees of Teneo and its clients.
"Over recent days I have received countless messages of support from my colleagues, clients, public figures and people who know me well both personally and professionally," he said.
"Their support and trust is what counts the most. I will be forever grateful to them all."