Cara House, also known as Mount Cara, on Redemption Rd, is a 25-bed home to 15 current residents, opened more than 30 years ago.
The board of management has said that due to Covid-19 and a reduction in occupancy that it will be closed.
Staff gathered at the vigil insisted they had been told in early June that the facility would stay open, only for a reversal last month.
They had been told it would close in late August, staff claimed, but that deadline is now believed to be uncertain.
Families and staff have called on the HSE to step in and save the facility, which the health body says it cannot do.
According to company accounts for 2019, with statements approved by the board of directors on April 28, Cara House saw a surplus of €22,467 for the financial year.