The chairperson of the South/South West Hospital Group Board, Professor Geraldine McCarthy from University College Cork, has resigned.
In her letter to Minister for Health Stephen Donnelly, seen by RTÉ News, she said that when she commenced her role, over six years ago, there was "excitement and hope amongst clinicians and managers for a more devolved, modern and equitable" health service.
She said that despite the excellent healthcare delivered at the frontline by committed staff, "it is regrettable that the much needed Reform of the Health Service has not been delivered".
Prof McCarthy said that this includes the establishment of Regional Health Authorities, with autonomy over decisions, budgets and capital spend.
She said she had waited a long time for developments, led by successive Ministers for Health and Government.
"However recent information and my own experience tells me we are no nearer to the required Reform than we were six years ago," she added.
Yesterday, the Chief Executive of the Health Service Executive said the organisation is highly committed to implementing all aspects of Sláintecare, including regionalisation, integration and waiting lists.
Speaking on RTÉ's This Week programme yesterday, Mr Reid said that there is "nobody who wants to get into this change agenda more than ourselves and our HSE teams".