I have no idea what would be involved in starting a ferry service at very short notice, but it doesn't sound straightforward.
It is, though, a route that they run during the summer so I suspect the only significant hurdle to overcome is finding sufficient crew to run it at short notice as I'm not sure how their crewing arrangements work for Manannan (i.e. do they just take on extra staff for the summer?). Though considering the service starts on 27 March anyway I'd expect all of that will be in hand at this point so it probably wouldn't have been impossible to start up a few weeks earlier!