"We'll know more on Stephen as the week progresses, but he's going to be a bit of a doubt," admitted McLaughlin.
"We've got an extra couple of days as it is a Sunday game, which could stand us in good stead.
"He just rolled an ankle, it's swollen and it's probably worse than I thought. He is having a scan (on Monday) just to see how it's healing up. We'll keep our fingers crossed, but we're worried."
Ferris, one of Ireland's best players during the recent Six Nations, will not want to miss another Heineken Cup quarter-final. A knee injury kept him out of his province's defeat to Northampton Saints in Milton Keynes twelve months ago.
The 26-year-old forward remains 'hopeful' of being able to play against the Munstermen, having undergone some intensive physiotherapy over the weekend.
Ulster have experience of winning at Thomond Park where they claimed a 37-11 victory over their provincial rivals in January 2009. They also posted a 33-17 win over Munster at Ravenhill in December.
Previewing next weekend's clash, McLaughlin added: "We're under no illusions going to Thomond Park. We'll have to be very strong from a defensive perspective. We're a very dangerous rugby side when we are in control of the football.
"We're in an excellent position and, if we can perform the way we're capable of and keep it going for the full 80 minutes, then I think we can come away with a momentous win."