SU1763 : Trig point on Martinsell HillThis trig point was the first OS pillar to be constructed, in 1935. However the first from which observations were made was that near Cold Ashby in Northamptonshire
SP6476, on 18 April 1936. Perhaps this explains the comments by several contributors to
https://trigpointing.uk/ that this trig looks 'fat'; maybe later ones were a bit slimmer to save on concrete? (Of course in the Highlands many trig pillars were cylindrical, to reduce still further the weight of material to be carried up the mountains.)
Source:
Sheetlines 35, p48 (The Journal of the Charles Close Society for the study of Ordnance Survey maps). Of course these statements relate to GB only; I have no information about Ireland.