Staffing remains the biggest issue in the fight against COVID-19, according to Nursing Homes Ireland (NHI).
The organisation says 40 of the country's 570 residential facilities currently have clusters of the virus.
Earlier this week, a nursing home in Galway appealed for help following a major outbreak.
The major COVID-19 outbreak at Nightengale nursing home, with 25 out of 27 residents and all but four staff testing positive, meant just one nurse and one carer were available over 72 hours.
Tadgh Daly, Chief Executive of NHI, told Newstalk Breakfast with Susan Keogh that the high levels of community transmission of COVID-19 "pose an existential threat to all nursing homes".
He said: "What we've seen in the early period there were high levels of community transmission and that had very tragic consequences as we all know across the nursing home sector.
"Whereas we saw in July and August, transmission in the community becoming under control, clusters were closing every day.
"But clearly, because of the high levels of community transmission, we're now seeing the clusters going in another direction again which is a very regrettable move."
He said it was a "huge challenge" to ensure that situations such as those which occurred at the Galway nursing home this week do not happen again.
Mr Daly said: "There's around 570 nursing homes across the country, some run by the HSE and the vast majority private and voluntary.
"Thankfully at this point in time, there's about 40 open clusters so that means the vast, vast majority are now COVID-free and we need to remain that way.
"Of the 40 that have COVID, my information is that there's about four in some difficulty because of the large numbers of staff that are out because of testing postive for COVID.
"What we need to ensure is there's a rapid response in that situation, in those outliers, from the HSE because staffing is the single biggest issue now.
"Early detection is happening, but when you have large numbers of staff out, the only way we're going to address that challenge is by prioritising those in nursing home care and moving staff from one location to another."
He said the "key learning" from situations such as those which occurred in Galway is that nursing homes need to be integrated into the wider health and social care system.