Short answer - Railtec sheet number 5710.
Long answer - that isn't what I used - see the diatribe below...
I drew my own artwork for these, shaping each digit individually based on dozens of photos (OCD in action again...). Then I got them printed by Precision Decals.
I offered the artwork free to Steve at Railtec too, in case he wanted to produce them commercially. It was at that point I discovered that he does produce something similar - sheet number 5710:
https://railtec-models.com/showitem.php?id=11581
I have found it hard to search the Railtec website effectively for Irish items. You can only search for one word or phrase (no and/or functionality). Sometimes they are listed as 'Irish' or 'Ireland' but there are many Irish items labelled CIE, such as the broken wheels. However, a search for 'CIE' returns hundreds of irrelevant transfers - so many that the ones you actually want are off the bottom of the page, and there isn't a 'next' button... (FYI, broken wheels are in sets 5605, 5606, 5607). So that's my excuse for not finding this set on the Railtec website!
Anyway, I like my custom ones better. The differences in the shape of the numbers may be small, but I know mine are right. Steve's are pretty close though.
Anyway, back to 156, here's the prototype at Ennis in the late 1980s, about to haul me to Limerick. This is about 15 years later than the period my model represents; by this time the loco had been repainted and received unshaded numbers on the cab fronts, and the roundel moved down to the 'standard' position. Also it has an S suffix to the numbers on the sides, OHLE warning flashes, upward-facing headlight and the tablet catchers had been removed.