Even though I prefer modern image, as in today, I would definitely buy several versions of an Irish steam loco if released.
I reckon that to make it fiancially viable, as others have said, a variety of liveries would need to be available, including a preserved livery.
Have any of the preserved locos had different liveries while in preservation? If so, and if represented by the model, you'd appeal to those modelling different modern era decades, as well as those modelling steam era.
From a sales point of view, with the A almost upon us, and several more bullets threatened by the IRM lads, maybe, in general, people couldn't afford to invest in steam for a while.
If there were two or three different laminates available in say, a years time (I'm sure this was discussed at length by @jhb171achill in a thread on the scarcity of coaching stock), each in a couple of liveries, to compliment steam, the A's and more modern era layouts, that would probably bolster sales of a steam loco in two years time.
A loco without rolling stock or rolling stock without a loco will probably suffer from slightly depressed sales.
Laminates would present an ideal bridge between the A and a steam loco, if the runs were big enough.
Either way, it's an amazing step to take.
I'll pick up several of the GWR Manors just to support the lads, and hope to one day see an Irish steam loco.
For now, my Irish steam collection is quite limited.