Only 200 staff have come into the healthcare system through the Be On Call for Ireland initiative, even though 3,200 have bolstered the front line.
Around 600 people who were approved for induction have since notified the HSE they are no longer available, it was revealed yesterday.
Sinn Féin now wants a jobs guarantee for 2,500 staff, including those cleared though the Be On Call for Ireland scheme, as part of a proposed €1.9bn additional investment in health this year.
The party warned of a "tsunami" of hospital admissions this winter because of Covid-19, seasonal flu, deferred treatments and screenings.
Capacity will be reduced by up to 40pc because of the need for distancing and infection control.
The Government is preparing a further €2bn spend on health for the remainder of the year, but Sinn Féin's proposal would double that amount.
"Some 3,200 staff have come into the system through a variety of ways," Sinn Féin's health spokesperson David Cullinane said.
"But only a handful have come from Be On Call. The rest were either directly recruited or returned home from abroad.
"It is very frustrating to see a pool of 1,600 who are supposedly on call but have not been called.
"They're in a pool, they have gone through all the clearances, they are available to be placed in work and yet they are not in work.
"About 600 of those identified as suitable have since notified the HSE they are no longer available.
"We don't want to lose any more. In fact, we need to go and attract those 600 back again, if we can.
"We're saying that every single opportunity must be taken to get more staff - as does every opportunity to get more facilities, beds and physical infrastructure.
"We have a plan for reopening the economy, we have a plan for reopening schools. Now we need a realistic plan to protect Ireland's health."
Sinn Féin has proposed a €1.9bn plan the party says will protect capacity in the health service.
The health system is under pressure on several fronts, the party argues, through overworked staff, Covid care, non-Covid needs, catching up on delayed cases, a reduction in capacity and the looming winter flu.
"We frankly need a response like we've never seen before," Mr Cullinane said.
Frontline staff are at burnout and cannot continue to work overtime in under-staffed conditions, hence the need to recruit all persons willing and able, he added.
Sinn Féin also wants to provide free access to the winter flu vaccine for children up to 12, over-65s and all healthcare workers and vulnerable groups.