This is a proper day trip (not the usual discount "cruise"!) and seems to be available all summer - including during school hols. It's the cheapest I've seen it.
The small print is:-
Depart 10.30am from Holyhead on Stena HSS Fastcraft, arrive 12.50pm Dun Laoghaire*
Depart 8.30pm from Dublin Port on Stena Superferry, arrive midnight Holyhead.
Realistically 6+ hours in central Dublin
*Dun Laoghaire is the port on the South side of Dublin city and a bit further out than Dublin Port.
Do it on a Tuesday for £5 return, or on Sun-Thurs (not Tues or Fri) for £10 return per person. Price is for anyone 4 years and over. Under 4's free. Booked on-line with Debit Cards 48 hour in advance. £5 surcharge for Credit Cards. £2 booking fee for non-web bookings. No refunds. Can't be used with Clubcard offers. Ends 9th September.
FWIW If you are an EU national then there's no passport required - just ID.