I hate to post this thread, looking at all the lovely snow pictures on the forum, but I think there will be significant and possibly some locally severe impacts from a brief thaw and potential flooding.
All the models are now more or less in agreement on the timing and we can be fairly confident that a widespread thaw will begin in the west early 26th while across the east it will come more gradually through the later 26th into the 27th. With this thaw, there could be as much as 10-30 mms of rain. Now it's possible that some snow and some sleet, freezing rain will fall ahead of this rain across parts of the east and north, but that won't change the outcome, it will only add more potential for the eventual mess to come. To give some idea of the pace of change, expect -15 C Christmas morning, -5 C Christmas afternoon, zero by midnight, +5 C by mid-day 26th, and +7-9 by mid-day 27th in most parts of Ireland. There will be local variations during the warming period as cold air gets trapped in valleys. Somewhere like coastal Kerry would warm up a lot faster than somewhere like a valley in eastern Mayo.
The operative word here is mess -- few places will get a quick, no-nonsense thaw. Lower elevations of the southwest and west that have snow cover may get this, a quick return to "normal" Irish winter conditions with little more than some local flooding. Further up in elevation across the southwest and large parts of Connacht, Donegal, west Ulster, I would expect this thaw to take most of the 26th and the night of the 26th-27th to remove all but high elevation snow, and that with the potential impacts of road flooding, stream flooding, and mudslides in a few places, cutting off some roads.
Further east, where there is substantial snow, the arrival of milder air will be delayed by the slow retreat of the arctic high, and the main impacts will be delayed to the 27th although they could start during the night of 26th-27th. Given the depth of snow in some parts of Leinster and Ulster, everyone in a flood-prone location probably needs to be on high alert for locally severe flooding. It will all depend on how much snow melts in higher reaches of your local drainage, or how much rain adds to the snow melt in low-lying drainage areas. Just like the snow streamers, this will be largely a wait and see situation but better safe than sorry, I wanted to sound a fairly strong alert here and hope it won't be needed, since many readers of the forum might have other things to do tomorrow and Sunday.
Of course, the deeper snow well up the slopes of the local hills and mountains will likely not melt, at least not very much, and yet there could be snow slides in a few places so I would caution against expeditions into the hills on skis or any other form of transportation during this unstable period.
A potential hazard that may not be so obvious is that of roof damage or collapse. Flat roofed buildings, especially where snow has built up, can start to strain under the added weight of saturated snow and cave in. This seems especially true for modern-style office buildings and recreational buildings, most houses with peaked roofs should just see their snow sliding off when it thaws. Larger buildings tend to have a more resistant form of engineering design, the sort of building to monitor is where you have half a dozen shops or offices or residential town houses in a relatively new building that is 1-2 storeys and flat-roofed. In Canada, we've had problems with arenas (rinks) caving in during similar weather sequences, so any buildings like that would be worth checking.
This thread can become a meeting place for those with observations about the thaw and flooding, and those who want to offer specific alerts and predictions that are likely to be much more useful to you than my very general long-distance early warning.
For those thinking of travelling late 26th or 27th, expect delays due to fog, road slush, road flooding or ponding, and in hilly areas, possible mudslides blocking roads. Before that period starts, there could be problems with ice from freezing rain or sleet.