Are you driving up to the North? If so, do you have your car arranged? If you are driving, purposely avoid the "A"/ "M" roads, at least one way. Great scenery on the backroads, with adventurous / fun driving. Lots of local places for a lunch stop, too.
Have a Guinness, too. The Gravity Bar could ensure it's a good pint, but any public within 1 km should be great. Guiness is very different in Ireland than in the US (or anywhere else)
Bookstores-aplenty in Dublin. Tower Records, too. It can be quite a throwback, retail-wise.
There are lots of interesting places to eat in Dublin; most are expensive. We love KC Peaches. They have cafe sandwiches, pizza, and sweets. They also have hot and cold buffets. The biggest location is across from Trinity College, which gives you an idea of their prices.
If you ask a local for travel ideas, particularly day trips, you might get a narrow view. You can cross the island in 4 hours. We made the Rock of Cashel a day trip from Dublin, with plenty of time there--no sweat. (2 hour drive)
On your way up to Giant's Causeway, there's the Brú na Bóinne and Newgrange. Lots of GoT sites, too, if you like the show.
The scenery on the Wild Atlantic Way tops everything, but you will be getting dicey weather this late in the year.