With Jared Payne injured, Ireland winger Craig Gilroy is deployed at fullback, allowing Michael Allen to be called up to the left wing for his first European start. Andrew Trimble lines up on the opposite flank, while Paddy Wallace and Darren Cave continue their midfield partnership.
Up front, Neil McComb will also make his first European start after being named at lock in place of Iain Henderson, who drops to the bench. McComb is partnered in the second row by Lewis Stevenson, who received a call up to the extended Ireland training squad on Thursday. There are no changes to the front row which includes Rory Best, John Afoa and Tom Court, who has collected Man of the Match awards in two of his last three starts.
Nick Williams, who is out for eight weeks with a medial ligament knee injury, has an able replacement at number 8 in the form of Roger Wilson. Captain Chris Henry and the in-form South African Robbie Diack are the other loose forwards.
With Marshall elevated to the starting XV, Michael Heaney is on the bench along with fellow backs Chris Cochrane and Jackson. Rob Herring, Callum Black and Declan Fitzpatrick retain their places as front row replacements, with Henderson and Mike McComish providing further cover.
The match on Saturday will decide whether Ulster play home or away in the quarter finals, and it could be a difficult afternoon if past results are anything to go by. The hosts have won three of their last four home games in the Heineken Cup, but lost the reverse fixture at Ravenhill 41-17. Ulster have never won in 14 previous visits to France in the Heineken Cup; the closest they came was a 35-35 draw against Toulouse in 2000.
If you're not travelling to castres, watch all of the Heineken Cup games live at Laverys Bar (Bradbury Place) or The Pavilion Bar (Ormeau Road). Both bars have multiple HD screens and food is served from midday until 9pm. CLICK HERE for all of their sports listings.
Ulster XV & replacements v Castres Olympique, Heineken Cup Pool 4, Saturday 19th January, Stade Pierre-Antoine, (kick off 1.35 GMT):
(15-9): C Gilroy; A Trimble, D Cave, P Wallace, M Allen; R Pienaar, P Marshall;
(1-8): T Court, R Best, J Afoa, L Stevenson, N McComb, R Diack, C Henry (capt), R Wilson;
Replacements (16-23): R Herring, C Black, D Fitzpatrick, I Henderson, M McComish, M Heaney, P Jackson, C Cochrane.