Growing numbers of older people who are more vulnerable to Covid-19 are being inflected as the potentially deadly disease spreads further.
Acting chief medical officer Dr Ronan Glynn warned yesterday: "We are now seeing a higher proportion of cases in older age groups."
He was speaking as figures show 265 people over the age of 65 caught the virus in the first two weeks of this month.
The figures highlight how the virus is infecting people who have lower defences in fighting its effects, increasing the risks of death from it.
Dr Colm Henry, the HSE's head of clinical care, said: "We are now seeing an increase in (cases in) the over-75 age group."
It is impossible to contain the virus among younger age groups, he warned.
Health Minister Stephen Donnelly said: "The increased spread of the virus to older people is a concern. International evidence shows that more recently cases of Covid-19 have been increasing in younger age groups before then spreading to older people.
"We know that older people are more susceptible to the virus and this is one of the many reasons why it is so important that the overall rate of cases is brought down.
"While there is a lot of focus on the number of cases in Dublin currently, we have to keep the rate of Covid-19 low all across Ireland.
"We are seeing increases in cases in many counties, not just in Dublin.
"We need to slow the spread of the virus to ensure lives are saved, our health service is protected, schools and colleges can remain open, jobs are safeguarded and overall so we can live our lives to the greatest extent possible during this pandemic."
The number of people hospitalised with the virus, including those in the Mater and Beaumont, continues to climb, reaching 73 yesterday, up from 23 in August.
The numbers in intensive care have also risen from six in recent weeks to 14.
The Cabinet meets today to decide on a range of restrictions to be imposed on the capital, the latest hotspot of the virus, for the coming weeks.
There were 240 newly diagnosed cases of the virus yesterday and one more death showing no indication the problem is easing.
There were four deaths from the virus in August but 15 deaths so far in September.
Yesterday's cases showed 119 were in Dublin, 19 in Wicklow, 17 in Kildare, 16 in Donegal, 15 in Waterford, 10 in Limerick, eight in Cork, six in Louth and five in Wexford.
The remaining 24 cases are shared among Carlow, Cavan, Clare, Galway, Kilkenny, Laois, Longford, Meath, Monaghan, Offaly, Tipperary, Westmeath.
Dr Glynn said half of cases were in Dublin. "We are now seeing a higher proportion of cases in older age groups.
"Act now to save lives. Limit your contacts as much as possible. Assume you and those you meet are infectious, keep your distance and do your part to keep others safe."
Some 61pc of cases were in younger age groups under 65.