[tim333]

In Ireland all the rooms came with a substantial hole in the wall, mandated in case you decided to put something like a kerosene heater in your room, although the heating was electical. I generally had to block them off to stop the wind whistling through.

[CalRobert]

Right - I think it was to handle Carbon monoxide (and general ventilation). If you look at part F here I think current regs are 6500 square mm for most rooms https://www.seai.ie/sites/default/files/publications/Domesti...

(TBF Irish standards have gotten much better for airtightness)

[rsynnott]

That's more for ventilation to prevent damp etc (Ireland has humidity issues). Modern houses typically have MVHR instead.