Colm,
I´ve lived on the camino for four years now, and December and January are the most pilgrim-free months of the year, what with Christmas and then Spanish Three Kings´ holidays closing down the few pilgrim albergues that remain open all year round. (MANY of the places listed as "open all year" are kinda CLOSED for the last week of the old year and the first week of the new.)
Distances are very long between available stops, pilgrims are very few, and many shelters won´t fire up the furnace to warm just one or two pilgrims. Only the hardcore are out there this time of year. If you intend to go, do a lot of advance work, make advance reservations at hostels and hotels, and don´t expect low-price pilgrim albergues to be open, even if their adverts say they are.
Maybe things will be better this Holy Year, as a lot of folk will want to be in Santiago for the New Year´s Eve closing of the holy door at the cathedral. If you enjoy long days of walking in solitude and severe beauty, this may be the place for you!
You may want to contact Johnnie Walker of this board, he spent his New Year holiday this year walking the final 100 km. of the Frances. His short film "Camino Hogmanay" may be instructive.
Good luck to you.
Reb.