I'm with #1, more or less.
If you're not sure that you're ever going to get back to Ireland, it might well be a shame to miss Dublin. It's a great city with sites deeply linked to Irish history, like the General Post Office, Kilmainham Gaol, and Dublin Castle, as well as some great cultural institutions such as the National Museum, Chester Beatty Library, and Hugh Lane Gallery and the Gate and Abbey theatres.
Dublin also has some really fabulous scenery right on the edges of the city, but if you're spending four days in Dingle beforehand, that may not be such a consideration in your choice of city.
But I would say that two days is a minimum.
Cork is a very nice town, lots of nice pubs, some pretty appealing districts, some good restaurants, and a great brewery-bar (Franciscan Well). If you just want a taste of Irish city life, then while it has a very different feel from Dublin, it would be just as interesting and as much (more, some might say) fun, but certainly with less historic attractions and cultural options.
You don't say what time of year you're going. Beware of the Cork Jazz Festival (26 to 29 October this year), which leads to massive crowding around the city, unless of course you like jazz, in which case you should check out the events and book tickets for the ones you're interested in well in advance.