The RPSI heritage set alone has a couple of Park Royals (c. 1955), a 1449 series CIE built open second (pre-)laminate (1958), and buffet car 2421 (1956) all sitting on 61'6 chassis. 1916 is not one of the original 1950s built brakes but was converted in the '70s. Originally it was a a 2162 series suburban composites (1956), the external and internal structure of which is well known and wouldn't take a lot to reverse engineer the conversion to its original composite condition.
Some earlier Bredin designed GSR-built coaches also exist in preservation from the Cork-Dublin expresses or the Cork-Rosslare boat trains but these are on 57', 60' or 66' chassis some on 6 wheel bogies which will not see the light of day, although they may be some hope for the former group due to the longevity as primary and secondary stock.