Prices will definitely go up after the December 12th draw but the cabins will book up quickly and sleeping on a sofa, if you can find one!! for a 16 hour crossing wouldn't be nice!
We went with Stenaline Wexford/Cherbourg last July and their about the same still at the moment. If you book by February you get a free Dublin/Holyead crossing as well - which reminds me I must use that!!
I'd recommend Irish Ferries over Stenaline, I'm going to try Dublin/Holyhead - Portsmouth/Le Harve next time as it avoids London.