The 18-year-old, who was from Dublin, was last seen alive at a hotel in Londonderry on Thursday 14 March, with his friend Alexandra O'Brien.
The 21-year-old woman's body was recovered from the Foyle that evening.
Described as "vulnerable", she had travelled to Derry after checking out of a hospital in Dublin the day before.
Rescue crews had continued searching for Kieran every evening at low tide since he went missing.
Teams from Foyle Search and Rescue, Coleraine Coastguard, Irish Coastguard and civil air patrol, Skywatch were involved in the operation.
Foyle Search and Rescue crews were out on Thursday when police alerted them about the discovery.
"Kieran's family have thanked the PSNI, Foyle Search and Rescue and Irish Coastguard for all their assistance," a police spokesperson said.
Craig Smith from Foyle Search and Rescue said it was one of the most tragic incidents the team had encountered.
"From the very start we have been searching at low tide every day," he explained.
"Then during the weekends once a week, we had what a full search which would include family members, members of the shore patrol, Foyle Search and Rescue and the boat crew and sonar teams assisted by the Sligo 118 rescue helicopter and PSNI helicopter.
"It's been a long going search, every day it takes its toll, especially with the charity being voluntary, trying to ensure that the river's being covered 24 hours a day and trying to keep our own emergency service available for the city, it's been long and tiring."
Mr Smith said every week ten to 15 relatives of Kieran were in Derry to help with the search.
"It was such a hard task for them, especially with them living so far away. But they passed on their sincere thanks and gratitude to all the volunteers ensuring that everything was being done even when they weren't in the city."