The excellent view of Snowdonia that Bunsen7 and others had from Lugnaquilla on Sunday 19th December 2021 was partly due to a distinct temperature inversion over Ireland and Britain on that day. This meant that the air at 900 / 1000m above sea level was considerably warmer than the air at lower altitudes. (At midday there was frost on the valley floor at Kelly’s Lough while on Cloghernagh and Lugnaquilla itself, it was almost tee shirt weather.) One of the effects of such a temperature inversion is that light is refracted and bent in such a way that it is possible to see a little farther around the curvature of the earth than normal. This, combined with the exceptionally clear air at 900 /1000m, meant that not just Yr Wyddfa (Snowdon), but also many of the lower hills of Snowdonia, were clearly visible from Wicklow.
It is 160 kms / 100miles from Lugnaquilla to Yr Wyddfa (Snowdon) as the crow flies.
https://www.freemaptools.com/how-far-is-it-between-Lugnaquilla_-ireland-and-Snowdon_-wales.htm
Horizon distance from Lugnaquilla (925m) is approx. 109 kms and from Yr Wyddfa (1085m) is approx. 117kms. Given perfectly clear air then it should, in theory, be possible to easily see Yr Wyddfa from Lugnaquilla. Perfectly clear air is a rarity however. Haze generated by dust and water vapour usually intervenes to obscure hills that should, in theory, be visible. The temperature inversion combined with the exceptional air clarity that day provided ideal conditions and memorable views. Linkback: https://mountainviews.ie/summit/13/comment/23372/