Looking at the 18z GFS model run, the signal continues to intensify for a very strong arctic outbreak starting around Thursday on this run, writes iWeather Online's Peter O'Donnell. 
(A Detailed Synopsis Of The Cold Spell In Ireland HERE)

The high pressure system which made way for the thaw and is now slowly chilling overhead is stretched out and destroyed by sudden events to the north where the Greenland high captures the height anomaly from the eastern Atlantic, and a huge polar hurricane (can't describe it as anything less) forms near Svalbard and drops rapidly south. 

This will have to be watched because as impressive as the current maps appear, if this feature were to hold together somewhat longer it would engulf Britain and Ireland in a very strong northerly of sub-freezing temperatures which is almost what the maps show now ... the only difference being that the northerly is rather strong instead of very strong.

I can't stress strongly enough that this will be a major winter weather onslaught and not just a drop in temperatures with a few local flurries.

This is like turning the eastern Atlantic into the Great Lakes, almost, when you have this much cold air rushing south this fast, it overcomes the normal tendencies of the Atlantic to modify air masses and turn streamers to mush. This setup could give blizzard like conditions even with winds somewhat west of due north, in many parts of Ireland. And we aren't that confident that the setup is perfectly modelled yet, what's probably equally likely at this point is that the polar low will drop into the North Sea and pull Siberian air across into the N-NE flow on following days. There is also a lot of super-cold air waiting to head west from central to northwest Russia and around Novaya Zemlya.

When this polar hurricane develops, winds are going to be 50-70 knots around Jan Mayen (volcanic  island  1000 km west of Norway, 600 km north of Iceland) with temperatures of -12 C ... the ice margin will probably shift south in this period to reach limits not seen in recent years, and the Baltic Sea is rapidly freezing and spilling out 0-2 C water into the North Sea. 

All of these factors are building blocks towards what may become an epochal cold spell for Ireland and Britain in the next 20-30 day interval.

Further information to follow.