Totally right, Mayner - that one's the same as the initial one, vents in side too. I believe they used ex-gunpowder vans for these gennys as they had more solid sides, which might drown the noise a bit. There is a standard "H" van preserved at Downpatrick which has a generator inside it, but that was fitted by the DCDR. When in operation it is known as "Roaring Meg" with good reason - you'd probably hear it in Australia!
It's clear that CIE converted a few of them in this way, possibly as the need for taking explosives by rail diminished. I wonder, though, if CIE actually BUILT any of them new for this purpose? As Mayner says, the standard design of van went back to GSWR days, and until CIE stopped building standard vans the basic design didn't change.