Never been to Ireland (would like to) so I don’t know how your climate compares to the different areas of the USA. I’m guessing you’re more like the North West Coast of America/Canada. They’re cold and wet, but miss out on the bitter cold waves coming down from Canada.
Michigan has a continental climate which is mostly dominated by arctic air masses from Canada all winter long. We do get the occasional break as warm air can push up from the Gulf of Mexico for couple of days. It is usually pretty darn cold!
I winter my full sized colonies with top entrances and my nucs with bottom entrances (mostly). All my boxes are well insulated polystyrene. I really haven’t had any problems with my top entrance hives. There is enough bees in them to generate enough watts of heat to keep the boxes warm even with top entrances. I feel the nucs are a little different; with less bees, they don’t have as much room for heat losses.
My top entrances are not huge and there is NO bottom holes in those boxes. Any gas exchange/convection currents have to come and go through a modest top entrance of 9mm x about 100mm in the coldest of the winter. Some heat is lost, but without a chimney effect in place, the loss seems manageable IMO.
The nice thing I’ve seen with top entrances is the bees behavior is more in phase with the outside world. One a warm day, the top entrance hives get out and do cleansing flights while the bottom entrance ones don’t. This is probably due to the long thermal time constant in an insulated hive, it would not be as pronounced in a wood hive.