I have to disagree with JHB that there were just two CIE Greens - the one inherited from DUTC and a later lighter shade. I think things are more complicated than that.
Mr. D. Kennedy presented a paper to the Irish Railway Record Society which was published in their June 1965 Issue No. 37. The piece was entitled "Modern CIE Coaching Stock" and I would make it clear that coaching stock is all that is being discussed in this reply. Mr. Kennedy covered mainly the vehicles constructed by CIE and, inter alia, liveries applied. He acknowledged the assistance of Mr. Thomas Tighe of Inchicore ("Railway Tommy" well known in connection with the sadly removed railway at Malahide Castle) and Mr. Leslie Hyland of "Irish Railway News", so the credentials are good. Bear in mind the Journal in those days was monochrome, so Mr. Kennedy's word descriptions are all we have to go on, except where stated otherwise below.
JHB has pointed out that CIE adopted DUTC dark green with eau-de-nil lining, with examples of the colour cited at Headhunters and No. 800 at Cultra. Mr. Kennedy describes this as "dark bottle green" and it was applied to the first CIE-built coaches, Compo's 2124-9 of March 1951, 3rd's 1339-50 of late-1951 and Compo's 2130-6 of early-1952. These do not appear to have had any lining, but did have silver windows frames and a light green "1" on first class doors.
3rd's 1351-5 of 1952 were similar to above but with the addition of a narrow light green waistband. A quote from the article: "These were the last coaches so painted, but despite a number of mid-green liveries which followed, the light green band was retained".
The AEC railcars 2600 onwards started delivery in November 1951 and early members appear to have had the dark green, but not specifically mentioned by Mr. Kennedy. Delivery continued for some time, such that later members appeared in a lighter green. See a photo of 2657 at Waterford Manor in the first Tom Ferris colour album, which is standing next to a disused Clayton, still in dark green. The railcar's roof is well worth looking at too! The West Clare Railcars (286-9, later 3386-9) appeared in 1952 and also appear to have had dark green, but not mentioned by Mr. Kennedy.
The fun starts with 3rd's 1356-71 and Brake-3rd's 1904-8 of 1953 which were turned out in apple green, which was according to Mr. Kennedy "most unsuccessful; within a short time the numbers were hardly legible, and the coaches were almost impossible to clean".
Mr. Kennedy then goes on: "During 1953 and 1954 various other liveries, all shades of mid green, were tested on many vehicles, but all were unsuccessful". Sadly, the shades and vehicles are not mentioned, but those concerned would have been Buffets 2405-18 of 1953/4, Compo's 2137-61 of 1954 and 3rd's 1372-8 of 1954.
The introduction of the new Park Royals, 3rd's 1379-1418 of 1955 introduced "Brilliant green" a lined light green which "became the standard livery for the following six years" according to Mr. Kennedy, although I have my doubts, as absence of paint seems to have been more common. This appears to be the lighter green described by JHB. The next batch of Park Royals, 1419-28 of 1956 had the same livery but with the addition of "2" on the doors, the first coaches to have them, presumably connected with the change of designation of 3rd to 2nd class on 3rd June 1956.
Heating vans 3101-41 appeared in 1955/6, but were in unpainted aluminium. Buffets 2419-22 of 1956 have no livery mentioned by Mr. Kennedy - were they unpainted or green? 2nd's 1429-43 of 1956 were unpainted aluminium, complete with a red "2" on the doors. Such was the lack of wear of this finish that Mr. Kennedy notes repaints in green started in 1958...
Bulleid Railcars 2660-5 of 1957 had unlined brilliant green according to Mr. Kennedy, but Compo's 2162-71, Luggage vans 2700-65 and 4-wheel TPO's 2962-71 of 1957 were unpainted aluminium, as were bogie TPO's 2972-8 and 2nd's 1444-8 of 1958.
2nd's 1449-96, produced from October 1958 to 1960 were green. Mr. Kennedy notes those produced after 1958 had a light green CIE emblem "a revival of the pre-1950 practice, which was continued until the present (sic) livery was evolved in 1961".
Brake-2nd's 1909-13 of late-1959 were green too, as were Heating vans 3142-52 of 1959/60. No mention is made by Mr. Kennedy of the liveries applied to Luggage vans 2549-58 of early-1961, Kitchen 2401 of June 1961 and Compo's 2172-5 of later-1961, but this comment suggests they were green. "Shortly after the release into traffic of the 2172 class compos, several coaches appeared in a new and very striking livery of black (upper panels, roof and ends); golden brown (lower panels); and white (a band just above window level)".
He duly notes new Compo's 2176-9 of 1962 as being the first new coaches in the new livery.
Mr. Kennedy does not say what livery was applied to existing coaches on overhaul/repaint, but presumably the prevailing shade of green would have been used?
As I've said before, "CIE Green" offers great opportunity for debate and dispute!