Carroll (pictured) left the organisation earlier this month to become head of policy and programme implementation in the Department of the Taoiseach.
Leo Varadkar took up the role of Taoiseach on 14 June - and the Irish Independent reports that Carroll "played a leading role in Mr Varadkar's leadership campaign".
Carroll had previously spent three and a half years as a policy adviser to Varadkar in the Department of Transport, Tourism & Sport, before leaving to join the PRII as CEO in July 2014.
While still at the PRII, one of his priorities has been the sector's compliance with and response to Ireland's new lobbying register, which Carroll said last year had been a greater success than its Westminster counterpart. His time at the helm has also seen membership grow by 20 per cent.
Development and research manager Frank Condon has taken on the role of interim general manager of the institute pending the appointment of a new CEO. The recruitment process has begun and is being handled by Dublin executive search firm Principal Connections.
Cian Connaughton, president of the PRII and a director at the agency MKC Communications, said: "This is a particularly exciting time to be working in the public relations and communications sector. Our Institute is in a great position to keep growing strongly and improving the events and professional development offerings for members. We are also better placed than ever to advocate on behalf of the profession and industry."
Carroll featured in PRWeek's Global Power Book 2017, and nominated recently departed White House press secretary Sean Spicer as the world's most influential PR.
I nominated @seanspicer - as for, good or ill, he is more than anyone else, shaping the popular perception of PR right now— John Carroll (@johnjcarroll) June 22, 2017