When making chairs of this ilk, I like to glue the entire backs together as separate assemblies. I then repeat the process with the front legs/seat rails and finally take the side seat rails and remaining stretchers and glue the whole lot together.
I dry-assemble the stiles, back seat rails and backsplats in order to establish the shoulders for the tenons on the tops of the stiles and splats. That done, I form the tenons and cut the mortises in the crest rails to match (figure 1).
Fig. 1. Chair back components.
Each component is shaped and rough-sanded except where they intersect with another. As can be seen by the pencil marks (click the above image to enlarge it), the junctures are left oversized until glued and assembled whereupon all are faired.
As with these two chairs, it is common for Irish chairs’ splats to be tenoned directly into the back seat rails (as opposed to into fixed shoes) and the shoes – more like ‘slippers’ – simply slip into position against the fronts of the splats (figs. 2 & 3).
Fig. 2. Slip-on shoes.
Fig. 3. Irish splat and slip-on shoe. (Windsor House Antiques)
Each shoe is retained with a couple of headless brads.
Jack Plane
Pegs and 'Tails 
I would imagine, the slip-on shoes would make upholstering easier.
Happy Holidays to you Jack Plane!
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Actually, the shoes are nailed on prior to the chairs being upholstered. However, in the past, when reupholstering period chairs, I have removed the shoes to avoid inadvertently striking them with the upholstery hammer.
JP
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I trained at the Edward Barnsley Workshop, their advice on back bars or splats is to make the crest float by a couple of mill, so that you’ve got enough slack for two or three rounds of scribing, thus ensuring multiple M&T’s will all close up tight together. There were a few options for delivering floating crest rails; tenons that are short in their mortices, fully blind tenons, or jointing the crest rail to the back legs with bridle joints. I’m guessing this ingenious technique, which is completely new to me, delivers much the same benefit?
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That all sounds perfectly logical. I’m afraid I am far too impatient for that approach though; I scribe once, cut, glue and fair.
JP
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