A friend and I are planning a trip that begins in Dublin the end of March.  We are then travelling on to London for a few days.  I came across the RailnSail option.  Irish Ferry to Holyhead and then a train to London.  It seems like a nice way to see more than if we just did airplane hops.  Any advice on this?  IrishFerry site link

Also further plans are to go to Edinburgh or Glasgow (undecided at this point) before heading back to Dublin for the trip back to the states.  Any thoughts on that would be appreciated as well.  Thanks

Edinburgh would be my choice out of the two, its nicer.
Be careful in london.
Atb

Thanks for the opinion.  I am leaning to Edinburgh myself.

If you know what dates you will be there, book your train tickets, they will be alot cheaper in advance.
Atb