Excellent news! I very much look forward to following this thread!
The mail train was indeed typically three to four six-wheelers in the period you have chosen. All three classes were catered for - at the least, a 1st / 2nd compo, a third and a full brake or brake 3rd. The Achill line, in early days, usually had a full 1st, full 2nd and full 3rd, with either a full brake (birdcage) or brake 3rd.
If you move forward into GSR times, a branch LIKE Belmullet would very often have a single bogie composite and a six-wheel brake 3rd.
The Alphagrafix kits are, I feel, very nice items for budget and they "look the part" for many prototypes which have no other alternative to total scratchbuilding. As can be seen with their CIE green stuff, though, with its WHITE lining and "snails" instead of light green, livery details on these are unfortunately only correct on about 50% of their catalogue, and some are complete flights of fancy! The birdcage brake in the above catalogue seems to be in Isle of Man loco green, with (British) Southern Railway gold numerals, not remotely close to anything that ever ran here......anyway; rant over;
Your "D-bogie" or "D16" will, given its provenance, be a great delight to behold, especially in lined Midland green.
The Midland also had some very fine 60ft full brake parcels coaches which were used on mail trains. They were bogies built about 1901/3, as far as I can recall without access to archives as I've all my stuff packed up in the process of moving house!