Yes, up until the introduction of the Mk II A.C. stock in 1972 all coaching stock vehicles could operate in service with each other. The Mk II A.C. stock could , as BSGSV says above, operate with non-A.C. stock but only one of the types could be in service (e.g. an A.C. train could be mixed with out of service non-A.C. stock or vice versa). Some of the pre-CIÉ stock lasted a long time. For example, GS&WR 3rd Class coach 845, which was introduced in 1907, was not withdrawn until 1971 so it could have operated alongside Park Royals, Cravens etc. The former State Coach, No. 351, had an even longer life having been built by the GS&WR in 1902 and not being withdrawn until the mid-1970s. The use of the State Coach (along with a GSV to provide power and heat) on trains composed of A.C. stock was an exception to the normal practice of only having one portion of mixed trains in service as the President could travel in 351 while regular passengers travelled in the A.C. part of the train. Having no connection between the two parts of the train was of course a security plus.