If you are only going to be in Ireland for 3-4 days you could probably spend them all in Dublin, spend a couple of days seeing the city, then maybe just do a couple of guided day tours say to Newgrange (north), or south to Kilkenny and Rock of Cashel, or maybe Glendalough. This will save you the hassle of renting a car, and with such limited time the day tours can actually maximize your sight seeing time as you won't have to waste time getting the car, finding your way, packing up and moving on. If you really want to get out of Dublin for a day then Kilkenny is easy by train, or even Belfast (I travelled all over Ireland by train, sometimes bus). Do a google search for day trips from Dublin and see what interests you as these tours sometimes book up so it may be wise to book in advance. I would certainly start looking for accommodation if you don't have anything booked yet, as you are travelling in peak season (the same goes for Scotland). As for Scotland I can tell you that I spent 6 days in Edinburgh, and did two guided day tours and spent the rest of the time in the city, then rented a car and did a ten day loop tour all the way up north then back down to Glasgow where I spent four days. Again, the day tours can be a good way to go when you have limited time, and there is a lot to see in both cities that you won't be bored if you spend a few nights in each. From Dublin my favourite day trip was the one to Newgrange and other sites. From Edinburgh my favourite tour was one that went to Hadrian's wall and Rosslyn Chapel. From Glasgow it was the trip to Stirling Castle and Loch Lomond. But there are many other options.
