18 May, 11:09
IRFU Chief Executive Philip Browne has issued a statement following confirmation of Ronan O'Gara's decision to retire from playing professional rugby.
Lock Neil McComb and hooker Nigel Brady also return to the panel for the European opener against the French club.
Number 8 Nick Williams is in contention to play despite having suffered a rib injury in the 25-0 derby win over Connacht. He is responding well to treatment and a decision on his fitness will be made later in the week.
Prop Declan Fitzpatrick aggravated a medial ligament injury in training last week and remains unavailable for selection.
Meanwhile, Williams' back row colleague, Sean Doyle, suffered a broken leg playing for Dungannon in the Ulster Bank League last Saturday and will be out for up to six weeks.
Tickets for the Ulster v Castres Olympique match are still available. To purchase tickets, please call the Ravenhill Ticket Office on 02890493222 (option 2) or visit www.ulsterrugby.com.
ULSTER Squad (v Castres Olympique) - Backs (11): Ruan Pienaar, Paul Marshall, Michael Heaney, Paddy Jackson, Paddy Wallace, Darren Cave, Luke Marshall, Andrew Trimble, Tommy Bowe, Craig Gilroy, Jared Payne.
Forwards (16): Rob Herring, Rory Best, Nigel Brady, Neil McComb, John Afoa, Tom Court, Adam Macklin, Callum Black, Johann Muller, Dan Tuohy, Lewis Stevenson, Stephen Ferris, Mike McComish, Iain Henderson, Chris Henry, Nick Williams.