Further to the discussion re the Coed y Glyn Granite Mine, in 19thC commercial terms granite was used for lots of hard rocks which were definitely not granite in geological terms, even some hard sedimentary rocks (eg Ingleton Granite - actually a greywacke).
Re mining of igneous rocks, this is understandably unusual. There were underground mines working whin dykes (dolerite) in Co. Durham & North Yorkshire, and some working elvan in the Bissoe area of Cornwall after the First World War (some opened by the Wellington Brothers who also opened Mount Wellington Mine). In view of the abundance of elvan in Cornwall I can only assume these were looking for particularly good quality stuff for building or ornamental use.
Denbighshire was recorded as producing 137,971 tons of igneous rock from mines (as opposed to quarries) between 1898 and 1907, which probably came entirely from Coed y Glyn (Ceiriog). Would this be comparable to the extent of the workings shown on the plan?