The only thing which should be indicating in the thousands on departure from anywhere in Ireland should be the altitude pre-select! The altitude initially cleared to by ATC will normally be set prior to departure.
Changes in local altimeter setting only account for much smaller increments of approximately 10' per hundredth of an inch hg or about 30' per hectopascal change in altimeter setting. I've seen it change by as much as 400' overnight but even that was a pretty significant weather front passage. Large tropical cyclones can produce much more dramatic pressure gradients.
Radar altitude will read somewhere near zero while the aircraft is on it's wheels unless malfunctioning.