Sorry, I forgot to update this with the “solution”! 
Everything was actually working fine: the “trick” is just the fact that the IRQ Traces come on different “flows” to the main application - so I just needed to enable those “flows” in the TMT (or M2MStudio Target view)
The “flows” are:
ADL - for the main application - as usual;
LLH - for the Low-Level Handler;
HLH - for the High-Level handler.
See the description of adl_trcPrint in the ADL User Guide.