temperatures in the Wicklow mountains
I know what you mean about how it was colder generally in those days and winters were certainly colder, that's true.
The Wicklow Mountains have long been lived in though and there was a British Army presence there because of the rebel hideouts which they were always trying to find. There are also a few great 'estates with the vast country house and some smaller but significant gentlemens' residences, so measuring, observing etc. could have been going on there at least on an amateur level but I suppose there may be no official records.
However, the Wicklow Mountains, as mountains go, are not especially cold or snowy. More than anything they're wet. It rains
alot. Mostly you only get very cold temps and heavy snow in certain parts and some areas are known for often being impassable for days at a time in December/January.