Griffin, who hails from from Knocknacarra in Galway city, is a former Ireland Under-18 international and a product of the Corinthians club and Colaiste Iognaid. He will be 19 next month.
The 18-year-old O'Halloran, a native of Clifden, was capped for the Irish Schools earlier this year.
He attended Cistercian College in Roscrea and helped Leinster Schools win the Interprovincial title last season. He began his rugby with Connemara RFC.
Both young players, who have been promoted to professional development contracts, will be in the squad for Friday's friendly against English outfit Harlequins.
There are four debutants in Connacht's starting line-up, with Miah Nikora at out-half, Brian Tuohy on the right wing, Bernie Upton in the second row and Mike McComish at number 8.
Connacht will be without most of their Churchill Cup players, including skipper John Muldoon, hooker Sean Cronin, scrum half Frank Murphy and centre Keith Matthews.
Coach Michael Bradley is pleased with the way pre-season training has gone for the westerners and is looking forward to seeing some of the new signings in action.
Connacht travel to play Gloucester on Saturday week before taking on Rotherham Titans at the Sportsground on Friday, August 28.
"Pre-season, as always, is about game-time so there'll be an opportunity for everybody in the three games," Bradley said.
"But the Harlequins game will be an opportunity for us to introduce a lot of our new signings this year.
"Our pre-season has gone very well and the lads have trained hard. We're happy with the intensity the players have shown and the progress they've made."
The Harlequins side will be captained by former Ulster and Ireland 'A' flanker Neil McMillan, while prop John Andress is another familiar face in the hosts' matchday squad.
CONNACHT (v Harlequins): Troy Nathan; Brian Tuohy, Niva Ta'auso, Aidan Wynne, Fionn Carr; Miah Nikora, Conor O'Loughlin; Ronan Loughney, Adrian Flavin (capt), Robbie Morris, Michael Swift, Bernie Upton, Mike McCarthy, Johnny O'Connor, Mike McComish.
Replacements (from): Andrew Browne, Kieran Campbell, Eoin Griffin, Jamie Hagan, Conor Higgins, Dermot Murphy, David Nolan, Ray Ofisa, Tiarnan O'Halloran, Brett Wilkinson.