Traveling to Shanghai on 2 passports - 1 eligible, the other not

I am both an Irish and South African citizen. I currently live in South Africa.
I plan to go to Shanghai for the CIIF 2017 and then on to Frankfurt before coming back to SA.

I am obliged by local immigration laws to leave South Africa on my SA passport. I'd expect that I apply for the 144 hour free transit based on my Irish passport, even though I am traveling on the SA one. Any thoughts of experience of this scenario would be appreciated.