A rise in the COVID-19 infection rate in Ireland is a "serious concern" but the country has not yet seen a significant resurgence in infections outside of identified clusters, a leading health official said on Thursday.

Ireland, which for several weeks had one of the lowest infection rates in Europe, has seen a spike since last Thursday and has identified a number of clusters of infections in meat plants and accommodation for asylum seekers.

The reproduction rate, or the number of people who become infected from each positive case, has increased to 1.8 from 1.3 a week ago, Professor Philip Nolan, the chairman of the country's Epidemiological Modelling Advisory Group, told a news briefing.

"A reproduction number of almost 2 is a serious concern, and although we have not yet seen a significant increase in community transmission, there is a significant risk this could develop over the coming days and weeks," Nolan said.

Ireland reported 69 cases on Thursday and the average infection rate has more than doubled in recent weeks to around 50 per day. It also reported five deaths on Thursday after six days with no deaths.

Acting Chief Medical Officer Ronan Glynn said half of all recent cases had come from three adjoining counties - Kildare, Laois and Offaly - and that a cluster of 60 cases from the counties were set to be added to Friday's numbers.

He warned people from those counties to be particularly careful and said he could not rule out specific restrictions being imposed on those three counties.