The health watchdog received nearly 300 complaints relating to COVID-19 in nursing homes in the space of five months.
According to one complaint, staff who tested positive for the virus but had no symptoms were asked to continue to work.
Freedom of Information files released to Newstalk show HIQA received the complaints between the end of February and last month.
They include concerns about a lack of PPE and poor social-distancing, and the effect the visiting ban had on residents' mental health.
It's claimed a resident escaped from a nursing home for a number of hours, without staff knowing, and when they returned, there were no measures in place to isolate them.
There were also concerns about two residents transferring from a unit that was coronavirus-positive to one that was COVID-negative.
There were complaints about numerous coronavirus deaths in some centres, a lack of testing and low staffing levels due to outbreaks.
One family member said they weren't told a nursing home had a suspected case of the virus when they admitted their relative.
Five staff in scrubs got into a taxi together after they left a nursing home, it's claimed.
A resident was allegedly neglected during the pandemic - they were left in bed, missed meals, had no clean clothes and had several falls.
HIQA says where it has concerns about residents' safety, or the care they're getting, nursing home providers need to take immediate action.
Nursing Homes Ireland CEO Tadhg Daly said the issues alleged in the complaints are "completely and absolutely inappropriate".
However, he told Newstalk Breakfast that it's important to point out they're also "unsubstantiated".
He explained: "They're issues that have been raised by relatives, by residents or indeed nursing homes themselves, whether it's public, private or voluntary.
"All nursing homes have a legal responsibility to investigate all of those concerns or indeed complaints... all of these would have been dealt with.
"HIQA in their statement made the point that if [the complaints] are substantiated, then they have the authority to deal with providers on a one to one basis."
He said people also have the right to approach the Ombudsman if they're not satisfied with the response to the complaints.
The HSE, meanwhile, says private entities account for 80% of nursing home beds in Ireland - but it has assisted them throughout the pandemic.
In a statement, the executive said: "This has been both nationally, through their representative body, Nursing Homes Ireland, and locally through individual providers and the Community Healthcare Organisation (CHO) network.
"Each CHO Area Crisis Management Team (ACMT) is prioritising the needs of long term care residents within their areas across public, voluntary and private facilities and has offered very significant support to the nursing homes in their areas.
"This has included management support, PPE, other supplies, Public Health inputs, other clinical inputs, support with sourcing staffing and, in some cases, the reassignment of HSE staff to support nursing homes."