It seems that water is drawn into the twig, saturating it, and the crust fungus exudes the water through pores. When the temperature, humidity and air are exactly right, the extruded water freezes and forms the hair-like structures we see in these photographs.
|'Hair Ice' caused by Exidiopsis effusa|
'Hair Ice' caused by Exidiopsis effusa
There is one previous record of the fungus from Ireland, made in 1993, but I suppose that must have been the original crust, rather than the 'Hair Ice'. So I have to settle for a second Irish record, with a Donegal first.
10 years for an id isn't too bad I suppose.
Coincidentally, we are experiencing the first cold snap of the winter, with lying snow and sub-zero temperatures. This was the view from my front window this morning as the sun rose behind falling snow: