The 20-year-old back, who is currently a member of the Connacht Academy, has now penned a full two-year contract with the westerners.
He earned his first cap for Connacht this season when he started against the Cardiff Blues in the opening round of the RaboDirect PRO12.
Marmion was born in Wales but his father and grandfather both hail from Loughrea, County Galway. He has been capped at Under-18, Under-19 and Under-20 level by Ireland.
He was an instrumental figure in the Ireland U-20s' stunning 23-19 win over hosts South Africa in the first pool game of the IRB Junior World Championship last summer.
Marmion played a pivotal role for the Irish youngsters during that tournament, starting in four of their five games en route to a highest ever finish of fifth place.
Marmion has gained all of his ten Connacht senior caps this season, being the only squad member to start in every match since the first competitive fixture back in September.
"It was a great experience to go over to South Africa and play against the best Under-20 players in the world," admitted Kieran.
"It gave me a lot of confidence to be chosen for the Ireland Under-20 side and then to start for Connacht this season was fantastic.
"I'm thrilled to have extended my contract with the province. I feel at home here and I want to continue contributing to the squad and the great work that is being done here every week."
Commenting on the latest contract extension, Connacht Rugby CEO Tom Sears said: "This is a fantastic boost for the province. This season we have seen the immense talent coming up through the ranks which is testament to the success of our Academy.
"Kieran is a key addition to the squad and we're delighted that he will continue to contribute to the future success of Connacht Rugby."