Garrett returned to the US in 1985 owning and running a contracting and development business primarily involved in loft conversion in Chicago, employing more 120 people. He moved back to Dublin in 1996 and created Shelbourne Development, which focused primarily on core Dublin projects - redeveloping urban brownfield sites. Before the 2008 economic crash, Shelbourne was active in the London, Paris and Brussels. Kelleher is based mainly in Chicago and Los Angeles while dealing with claims for almost €47 million sought by National Asset Management Agency back in Ireland.
Garrett Kelleher is the non-executive director of Lightstream Pictures. Lightstream produced MAX ROSE, starring Jerry Lewis and RAMPART  with Woody Harrelson. They are currently filming 2:22 with Michiel Huisman and Paul Currie. MOON AND SUN, starring Pierce Brosnan with Bill Mechanic which is in post-production.
At the request of Mayor Daley, Garrett was involved with the Chicago’s 2016 bid for the Olympics. In July 2006 Kelleher stepped in and bought a site at the intersection of the Chicago river and Lake Michigan after an earlier developer failed to close on the land. The 2.2-acre (8,900 m2) site would have housed the now cancelled The Chicago Spire. It would have been the tallest building in the US, and the world’s tallest exclusively residential building. However, Chicago's failure to obtain the Olympics meant the project had to be scrapped.
Kelleher sits on the Board of Regents at Ave Maria University in Florida. Opened in 2003 the University recognizes the importance of creating and maintaining an environment in which faith informs the life of the community. Kelleher also held the Chairmanship of Legatus in Ireland in 2006 which has aims of "To study live and spread the Faith in our business, professional and personal lives". 
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- Iona Institute