Those Beneteau Swift Trawlers are possibly one of the best boats to come out of the French manufacturer. I'm guessing from your fuel figures she has the standard Cummins 425hp 6cyl diesel.
I am a newbie here and in fact new to motor boating - but that is another story!...
From this intro line and the language you've used I kind of guessed you had a little more than some experience and a 1200 mile sail is not to be sneezed at.
Trouble is, when we were younger we were a bit more daring and hadn't experienced as much danger, at our ages though we don't move as quick and we worry much more about anything. 
Not a bad thing though.At about 7knts the Swift will be quite economical burning about 1 litre per mile so once in home waters you could take it easy. However the most obvious route from Greystones is a run down the East coast to Rosslare, then across to Milford Haven. It's about 114nm all in and at 15knts is about 7.5hrs. On the 4th March HW Dover is at 0826 so leaving at HW -1 will give you a bit of tide so SOG might be 16-17knts and then the stream crosses to Milford Haven to give you a good run in possibly with a bit of running against the ebb but not much.
From Milford Haven I would go to Padstow, its a great little Village and although the inner harbour gets a bit crowded in the Summer, they may have a berth for you. If not the outer harbour is a safe haven. It's only about 70nm so you could do a more sedate 12knts so 5.5hrs ish and be sparing on the diesel.
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I would then do Padstow to Falmouth another 86nm rounding Lands End at a safe time.
I suspect you have a Reeds Almanac, but if not, get one. Its' got all the information about harbours for this passage and tidal streams based on HW Dover times and a very good write up about rounding Lands End. The Channel Pilot does cover Lands End and shows a TSS to the west, it is easily covered by a Racon buoy so no worries there. If you want more information just ask and I'll look up some more.
I would say the main difference between Yachts and Motor Boats is with a Motor boat you don't so much have to rely on Gravy Trains (Tidal Streams) since most of the tides in the UK even at springs don't run much over 5knts and are usually around 2-3knts. However wind against tide is a different matter. I crewed in my friends Jeanneau Sun Odyssey 40 to Alderney last year in a F6-7 wind against tide some of the way and although it was a tad uncomfortable at times, especially making tea in the galley
it wasn't as bad as if we had been pushing along in a Motor Boat, wind against tide. So even though you are semi displacement and a superb hull shape bashing into a wind against tide sea would be a tad uncomfortable. Hope this helps a little.
