I stated in my Holyhead indtroduction page that I am sure Holyhead has much to offer and sadly I didn't have the opportunity to see much of it but I would suggest that the main reason for travellers to visit the town is to use the ferryport where boats leave regularly for the Republic of Ireland. This tip deals with that.
The Ferryport dominates the town and I will not add directions to it as you really cannot miss it. It is co-terminus with the train station and very well signposted should you arrive by road. If you are driving, there is a long stay carpark available with a complimentary bus service to and from the feryport.
The terminus is relatively small but clean and well-staffed with adequate facilities. There are toilets, a coffeshop, shop and a lefy-luggage office which was shut both times I passed through. If you are walking and want to go into town, stand with your back to the port, look towards the railway line and go out the door on the right over the causeway. Should you be going to the further part of the port for your ferry (the Stena part), there is a complimentary bus service to and from the vessel.
If I had one small complaint it would be that there is not really a lot of seating about but this is a minor point and the ferryport is generally a pleasant enough travelling experience.
The vast majority of people who go to Holyhead have absolutely no intention of spending more than a minimium amount of time they can. Holyhead is the main port for travel to/from Ireland and as such sees hundred of thousands of 'visitors' a year.
Two companies ply their wares between Holyhead port and Dublin City / Dun Lagonnaire (just to the south of Dublin). Fares will be very similar on both Irish Ferries and P & O Stena line. I must say that I prefer Irish ferries, which include the impressive Uylleses. The largest (or at least it was) catamaran car ferry in the world also operates from here in the hands of P & O Irish Sea. The quick ferry shoots across in an impressive 99 minutes whilst the more traditional ferries do the trip in a little over 3 hours.
Prices for using the car ferry can very somewhat, and are disgustingly expensive in high summer. At most times of the year a single ticket (car with passengers) works out at around 100 pounds. Take time to look through the websites of the two campanies as you will often find special fares or offers buried within them.
Foot passenger fares are more around the the twenty pound mark, and they should connect with train services through to London.
I have been to Hollyhead a couple of times to catch the ferry and Seacat to Ireland. From what I could see the port was the main part of the town.
The first time I went there I took the train and the second time I went by coach.