Connacht defeated Munster while Leinster got the better of Ulster as the four Irish Pro12 sides faced off in pre-season friendlies on Friday.
Ireland head coach Joe Schmidt said after the win over Wales in Cardiff earlier this month that the fixtures would provide another opportunity to run the rule over certain players.
Marmion was released from Ireland's squad to get more minutes under his belt and the exciting Connacht scrum-half capitalised on his pack's dominance upfront, scoring two tries.
Francis Saili made his debut for Munster along with former Gloucester winger Shane Monahan but it was Connacht who came out on top.
Nepia Fox-Matamua and a penalty try wrapped up the scoring for the visitors in a comfortable 28-12 win.
Meanwhile, Leo Cullen's time in charge of Leinster got off to a winning start since he was appointed permanently as head coach earlier this week.
Leinster edged out their hosts Ulster 24-21 at Ravenhill, holding off a fightback from the hosts towards the end of the second half.
Jack Conan returned to action for Leinster with a try after winning his first international cap against Scotland last weekend, one of four try scorers for Cullen's men.
Paul Marshall and Luke Marshall crossed for Ulster along with Ireland U20 prospect Sam Arnold, but it wasn't enough to complete a turnaround for Neil Doak's side.
Ireland's next Rugby World Cup warm-up match is against Wales on August 29.