^I said the sea near Dublin, and in fact the Irish Sea coast in general, is not that nice in my opinion. This thread is about opinions after all.
I accept that going to the beach is not really part of city breaks - you could just as well say Glasgow or Edinburgh have nice beaches near them, but that's not why people go there.^
The point about beaches and the coast in general is not that they are earth-shatteringly beautiful UNESCO heritage sites (though they are lovely) but that the coast, the variety of beaches and the coastal experience are something you won't find in Prague or Budapest or Warsaw or Krakow or Berlin or Paris or Vienna (yes there are river banks and river beaches and lakes in many places, but a sea coast is different).
They are a nice break from pounding the pavement, and if you want to sample an authentic Dublin experience - far more authentically 'Dublin' than a lot of the Paddy whackery that some tourist believe to be the real 'Ireland' or 'Dublin' - then a beach or a cove or a little harbour that has been there for a thousand years might be the place for you.
Part of the charm of Dublin, part of what makes it a nice place to live and to visit, and part of what distinguishes it from landlocked European capitals, is that you can experience raw nature right on your doorstep for the cost of a train or bus ticket. (I personally would place beaches within easy reach as a good reason to visit Edinburgh.)
Cities have their own individual character. Dublin was always a city of tenements, a port, a place where people eked out a living in trades, in brewing, in transport, in light industry, rubbing along together in close quarters. It was for over a century the increasingly dilapidated second city of the British Empire, a garrison city with the sort of culture and tensions that troops away from home bring with them. It survived through the poverty of the first half of the Twentieth century and has bounced back with aplomb while still retaining a lot of its character - it was never known for being squeaky clean (nor was it ever the Algarve of the North). Its realness and grittiness and dampness are the backdrop to everything that James Joyce wrote. Give me authentic any day over some sanitised Disney-type destination. You take the rough with the smooth in Dublin. That is part of what Dublin is.
Wrt 'dirty' - this song might even make me want to visit Salford, circa 1962.