IRFU chief executive Philip Browne says the financial crisis in Irish rugby caused by Covid-19 has particularly affected Ulster and Munster.
Browne said on Friday the professional game's "very existence" in Ireland was under threat unless fans could return in large numbers.
He also said Ulster's season of turmoil two years ago had left the province less equipped for the current crisis.
Ulster's financial position took a hit after the 2017-18 campaign.
During that season, a high-profile trial culminated in former players Paddy Jackson and Stuart Olding both being acquitted of rape.
Controversy over social media exchanges which came to light during the trial led to the pair's contracts being terminated by the IRFU and Ulster.
Ulster's difficulties 'well-documented'
Amid subsequent decline in ticket sales for matches at the Kingspan Stadium - including a drop in season ticket sales - Ulster's revenue suffered in the immediate aftermath and this was the context of Browne's comments on Friday to an Irish parliamentary committee, which he also touched upon at a later media briefing.
"Obviously in Ulster, they had their own difficulties the season before last that were well documented," said the IRFU chief.
Browne said Munster's "legacy issues" left them in a less robust position to deal with the financial carnage wreaked by the global pandemic, namely the debt following the upgrading of Thomond Park and "declining attendances" in their Limerick and Cork home venues.
By contrast, the IRFU chief said Connacht were in a better financial position to deal with Covid-19 while the dominant Irish province Leinster "have greater capacity to be able to withstand this sort of shock".
"Connacht have managed their resources very well. They run a very tight ship and the regime that's there has done a great job.
"Leinster obviously have the advantage of being the capital city but have managed to build up reserves over the years."
Further player pay cuts not ruled out
In the early part of the Covid crisis, players in the four provinces agreed to pay cuts to help out the IRFU and Browne pointedly did not rule out further salary reductions.
"We are in constant dialogue with Rugby Players Ireland," he said.
"Depending on what the prospective scenario is, when we views things forward in December, that will determine how we deal with the cost structures around the professional game."
The Irish provinces have been playing behind closed doors since the game returned last month and the recent increase in Covid cases throughout the island suggests this is unlikely to change in the coming weeks.
Browne expects Ireland's home Six Nations game against Italy on 24 October and their home matches in the autumn international tournament to be played without fans.
However, the IRFU hopes an improvement in the Covid situation could enable the halving of the two-metre social distancing requirement which would enable the Aviva Stadium to hold up to 18,000 fans if large crowds were again permitted at sporting events.
Some 12,000 of those tickets would be taken by premier seat holders, generating a badly-needed 32.5m euros for the IRFU's coffers.
"It probably leaves a balancing amount of in and around 6,000 tickets to sell. Six thousand tickets to sell isn't going to solve our problem but it goes a hell of a long way to getting things back on the straight and narrow."