According to Irish Ferries current (2009) SailRail leaflet (available from them and travel agents or as a PDF on their website) the times are Depart London Euston 2010 and change at Chester (depart there 2256) arriving Caergybi(Holyhead) 0111. This is marked as the connecting train for the 0240 Cruise Ferry departure. This sailing is usually highly reliable as it's operated by the MV Ulysses. Have a good trip.
PS: I suggest checking these times with
www.nationalrail.co.uk in case of any engineering work/bus transfers on any part of the route. Or e-mail Irish Ferries at
sailrail@irishferries.com
You probably know there's a Dublin Bus service which meets the ferry. Fare €2.50. All the best