Irish Ferries offer a �20 day return fare for foot passengers. You don't actually have to return on the same day as you travel out. You can return up to 24 hours after arrival (or on the first ferry after the 24 hours expires). You can travel on either the traditional ferry or on the fast ferry service. They both operate from Holyhead to Dublin City port. Timetable here:
http://www.irishferries.com/Timetable/gbr_irl. shtml
However, you shouldn't rule out ferry services to Dun Laoghaire. It's only 6 miles from the centre of Dublin and there are regular DART trains linking the port to the city centre. Stena offer combined rail-ferry-DART tickets on a 24-hour return basis for just �28 from Manchester. Alternatively, a return fare (without the 24 hour time limit) from London to Dun Laoghaire (including DART travel to central Dublin) is just �56. That might well be the most economical way of getting there. (As stated, it's incredibly easy to get from Dun Laoghaire to central Dublin. Even if you take a ferry to Dublin City port, you still have to take a bus into the city centre, so it's not really any quicker).
http://www.stenaline.co.uk/stena_line/stena_li ne_uk/holyhead-d_laoghaire/gb/by_rail.html
(Personally, I much prefer 'fast ferry' services to traditional ones).
Chris