A try scorer for Galwegians in their AIB League Division 1B win over Dungannon last weekend, he will make his competitive bow for Connacht on the right wing.
Connacht coach Michael Bradley said that the hard work being done at age grade levels in the province is producing some very promising young talent.
"Tiernan O'Halloran and Eoin Griffin have made the step-up to the senior squad this season. It is encouraging and hopefully it will go well for Tiernan against Olympus Madrid," he said.
With John Muldoon on the bench, Mayo man Gavin Duffy will captain Connacht for their first game in Amlin Challenge Cup Pool 2.
Bradley has made seven personnel changes to the side that lost 23-10 away to the Newport Gwent Dragons in the Magners League.
Niva Ta'auso, who has recovered from a hand injury, will partner Aidan Wynne in the centre and O'Halloran and top try scorer Fionn Carr come in on the wings.
Miah Nikora and Conor O'Loughlin have held onto the half-back spots, and Michael Swift and Bernie Upton will continue their second row partnership.
There are two changes to the westerners' front row - Brett Wilkinson and Adrian Flavin have been preferred to Ronan Loughney and Sean Cronin - and with Muldoon on the bench, Andrew Browne takes the number 6 jersey and Mike McCarthy moves to number 8.
CONNACHT (v Olympus Rugby Madrid): Gavin Duffy (capt); Tiernan O'Halloran, Niva Ta'auso, Aidan Wynne, Fionn Carr; Miah Nikora, Conor O'Loughlin; Brett Wilkinson, Adrian Flavin, Jamie Hagan, Michael Swift, Bernie Upton, Andrew Browne, Johnny O'Connor, Mike McCarthy.
Replacements: Sean Cronin, Ronan Loughney, Conor Higgins, John Muldoon, Dermot Murphy, Frank Murphy, Ian Keatley, Troy Nathan.