It's been a couple of years since I took the car to a Dubs game in Portlaoise, but I don't remember parking being a massive problem. The ground is about 1/2 mile away from the town centre, so there is lots of empty kerb space to park the car up on. Last time I went for the hurlers, it wasn't a full house. This time there will be, so that'll make a difference.
Best way for me, was to stay on the M7and pass the first couple of exits for Portloaise. Get off at the 3rd one and then follow the road back towards town and the ground. There are a couple of decent sized pay car parks in town, but they will probably be full with Saturday shoppers, so I wouldn't leave it too late to arrive.