That's a good tip, and a good port plan.
We should only meet the fast ferry at Kish Bank.
I think our initial course is 281 degrees, without allowing for any tidal flows.
69 nautical miles from Caernarfon to Dun Laoghaire harbour entrance at an average of 20 knots will just over three hours.
The other tricky bit by the looks of the chart is the traffic separation zone in Dublin Bay. Basically we have to cross it to get in to Dun Laoghaire, so your tip of miss the fast ferries, come definitely into play then.
On the upside I reckon from the South Burford cardinal you should be able to see the pub :0)
"By skill not force."