You could do a “Jackie Charlton” themed parade in which famous scenes from his life are acted out on different floats.
Jackie Charlton? would only take three floats 
Taking this one step further, how about characters from all Irish literature?
Lady Windermere with her fan, the Committments in their Van, the Third Policeman on his bicycle, barefoot Limerick gurriers for Angela’s Ashes.
I’d look forward to seeing the Beckett float, too, though you might be waiting for for a while for it to turn up… 
I like the Irish literature and drama idea. Everything from Gulliver’s Travels to the Boys (Borstal and Butcher). Loads of scenes with a strong visual component. Good craic but would your audience get it?
Irish sport? Horses, dogs, drunks, hurlers, prop forwards, music, colour, gambling, fighting…
Focus on a specific region? Each region has its own musical, literary and sporting traditions.
Me? A parade devoted to manwithaplan, genuine Irishman and egomaniac.
The new Ireland? Pluralism, technology and espresso, but still drink-sodden.
A journey through the counties? They can all be represented quite easily (you better go for 32 or I’m not speaking to you again).
There’s a whole scatter load of us who’d be more than willing to help out.
If you are going down the path of folklore and legends, you can get some ideas here:
Irish Literature, Folklore & Mythology…
Depending on how much work you are willing to put into creating the floats, you could do scale models of famous natural landmarks (Cliffs of Moher, Marble Arch Caves, Newgrange Neolithic Tomb, Giants’ Causeway, etc…) and specific areas of beauty like The Ring of Kerry, Dingle Peninsula, the lakes of Killarney, Horn Head in Donegal etc…
You could also maybe utilise the County shields in some part of your presentation.
I’d say that would be easy with all the guns you feckers have over there 
Hehe!
I’ve suggested the regions/counties and the lit/folklore to them. Anyone have any others?
If you go down the counties route, here are some good sites for reference information:
Flags Used in Northern Ireland (although this may be too political)
Province flags can be found here.
Or, if you wish to be controversial, you could use these.
Incidentally, if you are looking for further information on historical Ireland, this site can maybe help you out.
Let us know what you decide on and I’m sure we can help you find whatever information you need.