The INMO says two-thirds of nurses who have recovered from COVID-19 are still experiencing fatigue.
A new survey of 545 people found 497 (91%) stated they continued to experience symptoms - which also included mental health difficulties, headaches and breathing problems.
Director of industrial relations with the INMO, Tony Fitzpatrick, told Newstalk Breakfast: "I think the key thing to remember is people talk about COVID-19 and having an infection for 14 days or so and then it moves on.
"There's many stats out there that say that people recover after COVID-19 - but I think it's important that people realise that there's an awful significant number of the population that have contracted COVID that have post-viral symptoms.
"And from nurses and midwives what we're clearly showing in our survey is that they have fatigue, they have lethargy, they have difficulty with shortness of breath, they have issues with regard to their circulatory system."
"There's more and more evidence coming to the fore that shows that after you've had COVID-19 that there's a significant number of that population that then have post-viral symptoms."
He also said this is preventing healthcare workers from returning to work.
"It's important that politicians and the general population realise the impact that this virus has had on healthcare workers who have got COVID-19 - by the fact that the enduring legacy of the virus and the impact it's having on them now.
"We've people out for 12 weeks and more who've not been able to go back to work".
"I think people must realise that these people are going in and working on COVID wards, they have a greater exposure - particularly in nursing - to COVID-19.
"They are, for their 12 hour or 13 hour shift, working with COVID patients, caring for COVID patients through all stages of that illness.
"So that brings a greater level of exposure.
"When you drill down, in particular on COVID wards etc, where there may have been 20 staff nurses working on that ward, but 12 of those staff nurses have gotten sick, so that gives you an indication of the prevalence of COVID in those areas."
Meanwhile SIPTU has claimed that some healthcare workers may have failed to declare symptoms for fear of loosing overtime payments.
On this, Mr Fitzpatrick said: "From the perspective of nurses and midwives, I can categorically state that they did not attend work if they had any symptoms of COVID-19."
"But I can understand that other areas may have been under pressure with regards to the financial impact of it".
The INMO is presenting their findings to the Oireachtas Special Committee on COVID Response.
INMO General-Secretary Phil Ní Sheaghdha said: "Fatigue is a major risk to patient and staff safety - especially in a pandemic.
"Many of our members are reporting that despite recovery, they are still facing exhaustion.
"The impacts of this virus can be long-lasting, so nurses and midwives returning to work after recovery are going to need support.
"They will also need certainty that past mistakes are being corrected. The Government should empower the Health and Safety Authority to investigate cases."
She added: "As winter approaches, frontline staff face a toxic combination of fatigue and under-staffing.
"Safe staffing levels are the only way to ensure that our health service is not overwhelmed. We urgently need a clear plan to ramp up health service capacity before winter hits."