A single plant of Acer rufinerve, the Grey-budded, Red-veined or Honshu Maple from central Japan, is on the planting list at Dunorlan Park and I think must be the ID of the relatively broad-leaved Snakebark Maple planted in the half shade above the boathouse and the large oaks, beeches and limes on this slope.
According to the key, Acer rufinerve falls into the Macrantha group L on the key, those with 3-5 or 5 lobed leaves and membraneous pockets in the underside vein axils. This key makes no mention of the obvious reddish tufts of hairs also in the axils, which some authors say are persistent, as they appear to be here. Others claim that the hairs disappear in the summer, and the University of Copenhagen states that they are not found on the most vigorous shoots.
The overall mature leaf outline is said to always be obovate (quite broad?), as opposed to quite variable in a variety of other small Acers. The young shoot should then be blue-whitish bloomed to distinguish it from A, pensylvanicum which has a green young shoot, and usually longer obovate leaves. The petiole is greenish, as described by most books.
The plant in Dunorlan Park often look rather speckled with yellow and could perhaps be infected with a visible virus.
In July 2018 the developing fruits on this tree showed red where the seeds are, while the wings remain green. this is a feature of Her's Maple noted in the Collins Guide but Her's Maple (Macrantha Group K in the key) has its reddish hairs in the leaf axils soon lost, persistent white pegs, and stubbier side-lobes. Although it looks as though the seeds are developing well, the tree is dioecious according to Wikipedia, so the seeds will probably be infertile. This doesn't sound like an invasive species - see below? In addition, the sex of the trees is said to be malleable with environmental conditions according to researchers who worked in its native range in Japan.
The young stem is that lovely green-striped pattern that you see in the books - but the overall shape of the young tree is rather disappointing, rather too upright and ungainly I thought - and the potential space for this tree is a little cramped. so I don't think that will change. Note the greenish bud in this photo, not yet obviously white-bloomed.
Amazingly, this tree is said to show signs of invasive-ness in Belgium!
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