Maybe we'll see a 'project 61' but I won't built up my hopes. In the initial 1951-5 rolling stock building programme under CIE the first several composite coaches were built on the 60' chassis then in use for many years following the 57' chassis for bogie stock. Thereafter almost every coach built by CIE was in the new 61'6"" chassis, even the second 5 composites following the initial batch. The originals had 3F 4T compartment while they second batch had 4F and 3T compartments due to the longer chassis. Almost every CIE coach, thereafter, brakes, composites, compartment thirds, open thirds, were built on that chassis. Commonwealth bogies although some GSR style bogies seem to have been re-used.
Is there a link to the photo you posted or its your own @Wexford70 I just wanted to zoom in on the stock to see if possible to identify them better.