To do the whole journey by train, you will need to travel into England & then back out west from Shrewsbury. Fares and schedules for your date have not yet appeared on the system and engineering work may be taking place on a Bank Holiday. So, try this site in about 2 weeks to see what the situation is regarding your date.http://www.nationalrail.co.uk (Pre-booking is usually cheaper as prices are likely to rise as date approaches - so check for dates like tomorrow to see the difference - if any). Where engineering work does close lines, the train company will provide a bus alternative.
This site may be able to provide you with a cheaper foot passenger (on the ship) & rail fare combined.
A shorter bus route does exist but it probably does not run on a Bank Holiday. www.traveline.info
You may also wish to do a search on this site to try and find a car hire company that can deal with you on the Bank Holiday Monday at Holyhead.http://www.travelsupermarket.com/c/cheap-car-hire/uk/