Don't MCN usually trot out some promotional ferry booking codes around this time of year?
Don't know about the 'nearer the day' pricing, seems to be as obscure as airline pricing.
Mind you, it's nice to know that people like Ryanair sometimes get it wrong big time. A couple of weeks ago on Saturday I visited my son in Dublin and got an outward flight from Luton (which is only 12 miles away from here) for 5 pence, by booking a month or so in advance. Turns out that the Ireland Vs Wales rugby fixture at Lansdowne Road was on the Sunday and Ryanair could've sold all the tickets several times over at full price to the hordes of rugby fans (from both sides) who wanted to get on the flight.