Yes, indeed, Minister, it was. It started life on the Castleisland Railway Company - it is not known whether they had a livery of their own for their only locomogtive, but it would have been painted in GSWR colours after only a few years. The appropriate version of GSWR livery for that time is what it now carries in Downpatrick. The precise scheme was obtained from a large scale model built at Inchicore around tha time (of a GSW 2.4.0) and presented to the Institute of Mechanical Engineers in London, where it resides to this day, and may be viewed by appointment.
It would subsequently have acquired the later version of lining with the same dark green - this was cream and black. After about 1905 it would have been repainted black with red lining, and probably retained this after its 1915 rebuild into its current state. The grey started to appear about 1918 and quickly spread to most GSW locos, and to all GS ones after 1925. When 90 was withdrawn it was painted the way you (and I) remember it. This was a reonable approximation of an earlier GSWR livery, as carried on No. 36 in Cork station. That lighter green seems to have been replaced by the dark olive green about 1870 - before 90's time.
90 first went on display in its post-withdrawal scheme at Fermoy, and later to Mallow. It is DCDR's intention to keep it as it is. Basically the loco is in working order, but it needs a boiler lift for insurance and a number of small adjustments made before it re-enters traffic, though this will come abouit in due course. There are several youtube clips of it in traffic, one showing it fully lined in late 1870s style.