You've come to the right place, asking for tips! Naas is very very close to Dublin, it's about a 30/40 min drive from the city centre. And Oxegen takes place in Punchestown in Naas.
Now tbh, there's nothing really to do in Naas
At least I don't think so anyway. It's very small, has a few boutique shops but I think your best bet is to either go to Dublin or go to a different part of Ireland after Oxegen.
Killarney is quite a distance from Naas. It's about 280km. The drive will take about 4 hours or so. You can take a train though if you don't drive. The train will take the same amount of time, lol
Here's a route planner for Ireland
http://www.aaireland.ie/routes/default.asp
Here's the train website as well so you can get an idea of places the trains serve, etc.
http://www.irishrail.ie/home/
You should defo visit Killarney though, it's very nice and scenic there! Most people who come to Ireland visit Kerry, tiz great!
Oh I think Aer Lingus have a deal on at the moment for US flights but Continental and Delta also fly into Ireland from the US.
Oh and the best tip you will ever get for Ireland in July - bring a rain jacket! It tends to be a pretty rough month for rainfall.