Denis Hurley switches from the wing where he has impressed in the last three games to full-back, while James Coughlan should prove a more than able replacement for his Dolphin club colleague Williams in the number 8 slot.
The other change up front sees Wian du Preez replace the injured Julien Brugnaut (groin), while McGahan's bench has a decidedly youthful look to it drawn as it is from either Academy or Development players - the only exception being Tom Gleeson who loses out to Lifeimi Mafi in a reshuffled back-line.
With Ronan O'Gara amongst the players being rested as part of the IRFU Player Management Programme, Paul Warwick slips into the number 10 jersey and Mick O'Driscoll will captain the side in the absence of regular skipper Paul O'Connell.
Munster, although buoyed by their 35-3 bonus point win over Connacht last time out, will need no reminding of the type of challenge that they face in Saturday's sold-out game.
O'Driscoll said: "It's easily one of the toughest games of any season. Ulster have a great home record and besides that, last weekend without key players, they gave a full strength Leinster one helluva game.
"So yes, it's going to be very difficult to get anything out of this one. Always is in Ravenhill, but these are the sort of games you want to play in and I'd be happy with our form going into it."
Munster team manager Shaun Payne added: "Obviously the double loss to Ulster last season isn't forgotten in a hurry. Those hurt a lot.
"Ulster are an excellent side. They can attack you from anywhere on the pitch, they have exciting young players, some guys really coming through.
"Obviously they have players like (Stephen) Ferris, but I think (Ian) Humphreys as well is someone who is really doing well. They have (Darren) Cave in the centre too and a couple of really exciting back-line players and an excellent back row.
"We lost to them twice last season so I don't need to be talking them up at all and we know that the job that faces us, especially up there at Ravenhill.
"We haven't had a whole heap of success up there and this is always a massive game for us."
MUNSTER (v Ulster): Denis Hurley; Doug Howlett, Lifeimi Mafi, Jean de Villiers, Ian Dowling; Paul Warwick, Peter Stringer; Wian du Preez, Damien Varley, Tony Buckley, Mick O'Driscoll (capt), Donnacha Ryan, Billy Holland, Niall Ronan, James Coughlan.
Replacements: Mike Sherry, Stephen Archer, Peter O'Mahony, Tommy O'Donnell, Duncan Williams, Scott Deasy, Tom Gleeson.