It is my recollection that the C class were purchased for PW trains, not lignite. NIR ended up with more C class than planned because CIE were literally giving them away. We later discovered why LOL.
Regarding lignite, the plan was to acquire new wagons and locomotives (GM Co-Co aka 111 class) funded by the terms of the contract.
It never happened because of the adverse environmental impact.
The lignite traffic was being considered around the same time as nationwide flows of refuse to a mega landfill site at Maghermorne. Again it was stillborn because of environmental objections.