Murtagh was stable jockey for Oxx between 1992 and 2003, during which time he rode 18 Group 1 winners for one of the world's leading owners.
Most memorably he guided Oxx's Sinndar to victory in the Epsom Derby, the Irish Derby and the Prix de l'Arc de Triomphe in 2000, and is now back in the job following his resignation as No 1 for Coolmore last week.
"Myself and Johnny have had a very successful association over the years and when Johnny announced that he was leaving Ballydoyle, His Highness was keen to renew that old association," said Oxx, who has 90 horses in training for the Aga Khan.
Murtagh's appointment brings to an end Fran Berry's brief stint as Oxx's stable jockey. Berry was given the job at the start of the year following the retirement of Mick Kinane at the end of 2009 and although Berry will not be retained in 2011, Oxx hopes he will still ride for the yard.
"What has happened is no reflection on Fran Berry, who is one of the country's leading jockeys and is currently having his best ever season," the Currabeg handler continued. "We've had a great relationship and a very successful one and he'll continue to ride for the stable when the opportunity arises.
"Obviously Fran is not retained by the Aga Khan and presumably he'll be working as a freelance, but we look forward to him riding for us in the future."
Murtagh expressed delight at being handed the role. He said: "I am thrilled to be renewing my association with His Highness' horses in Ireland. We have shared some fantastic moments and I have always enjoyed riding in the green and red colours."
Murtagh becomes the first retained rider in Ireland for the Aga Khan, who added: "We have had some wonderful days in the past and we look forward to renewing our successful association. I believe it is vital to the continued success of my bloodstock operation to have a retained jockey in the countries where my horses are trained."