The chief medical officer at the Department of Health is undergoing tests in hospital.
But Dr Tony Holohan told RTÉ it's unrelated to COVID-19.
The Deputy Chief Medical Offier, Dr Ronan Glynn, is expected to take over until his release tomorrow.
Dr Holohan has held regular media briefings about the coronavirus over the past month.
He appeared unwell during a briefing on Tuesday, and went to St James's Hospital for treatment.