Looking for best value from
Holyhead England to Edinburgh..on to Glasgow..and then back to Holyhead.
A rail pas that's good for 3/4 days travel in a 2 week window. I will be using a ferry from Dublin to Holyhead on 6 Sept. 2016

Point to point tickets, singles/RT bought in advance, are almost always a better deal than any pass. Some fares have no discount, same price walk-up on the day, can be checked on national rail.co.uk.

http://www.nationalrail.co.uk

you can buy a through ticket all the way from Dublin to Scotland - more sense than buying a ticket just between Holyhead and Scotland

http://www.seat61.com/Ireland.htm

A pass may be viable economically only if you want to be able to hop any train anytime and not have to put up with the often severe restrictions on changing discounted tickets from a specific train you are booked on in some cases. For lots on British trains and passes check www.seat61.com- great advice on discounted tickets; www.budgeteuropetravel.com and www.ricksteves.com.

Seniors get discounts on first-class BritRail Passes and if not on a tight budget IMO go first class on long-distance trains - a world of difference IME in comfort level and usually empty seats if just hopping on -2nd class is often full.

It might be worth checking here: https://raileasy.trainsplit.com/main.aspx