Recently stayed at the Comfort Inn Smithfield. Brand new, near the light rail and a very pleasant place. Free internet, good breakfast, upscale grocery next door, within walking distance of everything. Be aware it is out of the way and in a redevelopment area that hasn't quite caught on. Very clean and quiet. I would stay there again and walk to Temple Bar and O'Connell Street, both of which are rather lively.

Be aware of the enormous number of buses rumbling up and down O'Connell and if staying at one of the many hotels there ask for a room at off the street. In Temple Bar I would want to not be close to a pub, if that is possible. (One hotel there says on their website something like Yes, it is noisy. Just deal with it.)

BTW, if they want to see a bit of the countryside, I highly recommend Over The Top Tours, particularly the Wicklow day trip with guide/driver Tommy.