The best option is to take the motorway towards Limerick until you get to the Exit 26 for Nenagh (N52). Stay on teh N52 until teh 3rd round-a-about and take a left onto the N494 for Portroe. In Portroe, a left turn will bring you to the quarry.
Keep left through the gate as you go in, then park in the big area on your right.
Portroe is a dive center. There's a shop at the site office, and they do tea and coffee, and you can get your tags for air fills there too. Kit up near the carpark, and head down the slope to the water. The area around the entry point can be slightly crowded, so be respectful of other divers, and wait your turn.
Generally, unless you get there early enough in the day the vis can be affected by other divers. Bear this in mind, and stay off the bottom as much as you can. Silt gets stirred up easily here. The other thing to consider is that there is a rescue boat in the quarry, so it's not impossible that it will be on the water. Use an SMB if you're not coming up the sloping path.
Also worth bearing in mind, for those used to diving in the sea, the fresh water will require less weight, and will cause your computers to register a shallower depth than you are actually experiencing.
Take your pick! Basically this site has something for everyone, there's 12m to the left of the entry point, which is ideal for practicing assessments. In this area is an old muscle car, a canoe and a white ford van, which can be fun to examine. Keeping the wall on your left and heading South South East brings you to a slope to 15m, where there are a few shacks still visible. In this area, near the South Easterly wall, there's also a sunken boat, the Shamrock, sunk on St. Patrick's day 2011. Behind the boat there are occasionally large eels to be found resting among the rocks.
Continuing past the shacks and the boat in a WSW direction will bring you to the edge of a sheer cliff. The top is at around 17-20m, and the bottom is at over 40m, so don't go over the edge too quickly! If you turn right at that ledge and follow the wall down slowly to around 25m, you'll find the remains of an old winch. From here, there's a path to the ENE that leads to a mineshaft. The path slopes upward, and the shaft is at 25m depth, so it would be a good idea to see tis at the start of the dive, to keep a good profile.
When you're done, go back to the Southern tip of the 12m section, and you can join a path that slopes upward to the entry point. Following this means that there's no need for an SMB, and makes it easy to regulate your ascent rate. There's even a flat area at 5m for your safety stop!
More than enough here for one dive, so break it up among however many you plan on doing. Bear in mind that, while it may be the only diveable place on a winter's day, the water tends to be a lot colder than the ocean, as cold as 4 degrees, so it's not for those sensitive to the cold.