The Kiwi’s departure was always expected given the financial problems that have hit Carnegie who are moving to a part-time operation next season.
And returning to Championship rivals Pirates - where he was high performance manager from 2009 to 2012 - is a perfect fit for Stirling who had moved from New Zealand to Yorkshire on a three-year deal only last March.
Carnegie, who lost 33-39 at Cornish on Sunday, finish off their season at home to Hartpury on Saturday with the player exodus also continuing.
Long-serving centre Pete Lucock joins Doncaster Knights on a two-year deal and Brendan Cope, the fly-half whose season was ended by a knee injury in September, rejoins Jersey Reds.
Stirling, 56, first worked for Cornish on an advisory basis in 2008 before taking up his role and helping them become the first winners of the British & Irish Cup in 2010, also leading them to a Championship final against Worcester Warriors the following year.
“My prior time in Cornwall, with the Cornish Pirates, was a key factor in both my coaching and personal development,” he said, going on to work as talent identification manager for New Zealand RU and in the football department at Super Rugby club Hurricanes.
“It is with great excitement that I return to the Mennaye to rekindle my working relationship with (coaches) Gavin (Cattle), Alan (Paver) and all involved at the Pirates.
“All involved in the rugby programme have achieved outstanding results over the last period and the future is extremely exciting. As Director of Rugby my objective is to assist with taking the Cornish Pirates to the next level of performance.”
Stirling joins on a three-year deal as Cornish - who hope to move into a new stadium in 2021 - step up their bid to eventually win the Championship and earn a place in the Premiership.