There are "yellow machines" at Downpatrick - vintage enough - but I'm pretty certain they are all of NIR origin. I'm unaware of any CIE ones anywhere.
jhb171senior recalled that during his watch, the GNR's first ever tamping machine was tried out on the INW between Ballybay and Dundalk at several places, as were concrete sleepers. All concerned murmured approvingly into their soup, so the contraption was deployed on the main line. This would have been about 1955-ish.
Incidentally, the trend of having all PW and maintenance equipment painted yellow is a comparatively modern phenomenon. Prior to 1980, while some was yellow, other stuff wasn't. Workers only started wearing day-glo in the late 80s. Prior to that, if anyone's portraying a maintenance scene on a layout, dungarees / overalls / donkey jackets are the only show in town. As are cigarettes in the workplace....
The old railway companies painted ballast wagons the same as their goods livery - i.e. inevitably grey. The GSWR used black.
(Additional note: the DCDR's plough van would originally have been all over black, with GSWR markings. Later, wagon grey (yes, chassis included) with GSWR markings. The GSR followed suit, as did CIE. It was never brown, as it now is, nor did it have a black chassis except when painted all-black. Just for info.)