Large areas showing high pressure are generally due to dynamic anticyclones e.g. the Azores anticyclone : the air is sinking (subsidence) and resulting in a compression which increases the temperature.
However there are also thermic anticyclones observed in winter due to the accumulation of cold air e.g. the Siberian anticyclone. Not sure that you get it in Ireland.
Some additional information:
https://content.meteoblue.com/nl/meteoscool/large-scale-weather/high-low-pressure
The explanation you give from this website is not wrong but as a general rule it applies mainly for local phenomenons (convection).
I may add that with high pressure conditions it is possible that you get a cold weather.
For instance if you are on the boundary of the anticyclone and you get a north wind, of course the temperature will not be mild. But it is due to the wind, not directly to the pressure itself.
Another reason for getting low temperature with anticyclonic conditions: they bring generally clear skies which allows the ground and the air layer near the ground to cool during the night (Clouds prevent this drop of temperature due to the radiative lost of heat). But this is due to clear sky, not to the high pressure directly. And during the day the temperature increases much more with clear sky.

"Since warm air is less dense and creates less air pressure, it will rise; cold air is denser and creates greater air pressure, and so it will sink."

I'm assuming a more in-depth knowledge of how weather works would be needed to reconcile the conflicting information.

There are a lot of good posts on this community explaining how to do it :
https://community.windy.com/topic/10336/customize-the-color-scales-of-windy-layers?_=1596358666767