Giant's Causeway is a spectacular rock formation on the Antrim coast of Northern Ireland. The site consists of some 40,000 basalt columns rising out of the sea. The Giant's Causeway is Northern Ireland's only UNESCO World Heritage Site.
Popular mythology attributes the creation of the Causeway to an Irish giant named Fionn mac Cumhaill. To prove his superior strength and status, Fionn decided to fight against a rival Scottish giant named Benandonner. As there was no boat large enough to carry huge Finn across the sea to confront Bennandonner, he built his own pathway of stepping stones from Ireland to Scotland. He then was able to walk across the sea without getting his feet wet.
When he crossed the sea, however, he saw just how large Benandonner was. He ran back to Ireland before Bennandonner saw him, but the causeway was built and Bennandonner came to fight. Fionn crawled into a crib and when Bennandonner came to the door to fight him, his wife told him not to wake the baby. Seeing just how large Fionn's "baby" was, Bennandonner grew afraid and ran back to Scotland, tearing up the causeway as he went to prevent Fionn following him.
There is bus service  from Portrush and Coleraine train stations, about £3 each way.
From Belfast there are day trips to the causeway which also include stops at Bushmills and the Glens of Antrim.
From Belfast, follow the signs for the "Giant's Causeway Coastal Route" for a beautiful scenic route to the Causeway. It takes a longer (around 2 hours depending on traffic) but it is worth it for the views.
There is also the more direct route along the A26 from Belfast or the A2/A37 from Derry/ Londonderry if time is a factor.
By sightseeing tour from Dublin
Several Dublin based tour operators such as Kennedy & Carr Travel  offer daily tours from Dublin city to the Giant's Causeway, often including other areas of interest along the way such as Belfast City, the Carrick-a-Rede Rope Bridge, Dunluce Castle and the Antrim Coast. Typically tours depart around 07:00-07:30 in the morning and return at approximately 20:00. Tickets generally cost between €60-69. For many travellers such tours can provide the independence they want with the structure and local insight that makes for a relaxing tour. Such operators also offer private hire services upon request both from Dublin and from Belfast.
Public transportation is scarce over the weekends, especially on Sundays after late September. Getting into Belfast, or even to the local towns and villages can be challenging.
Hiring a car, or making an advance note of a minicab company's phone number is recommended step to take.
The focal point of the area is, of course, the Giant's Causeway itself. There is no charge for visiting the causeway, although you will find that parking costs a little more than you would expect (circa £6). That being said, this is a free site (eg. no entry fee), so a car with four people works out quite reasonable. Cheaper still, check out the near-by Cuaseway railway carpark (circa £4) or park on the verge of the main road (free!)
Once parked up or off the bus at the visitors centre, you will find there is actually a further road that gets you down to the Causeway. There is a bus service runs constantly back and forth between the visitors centre and the rocks themselves, but walking there will only take around 20 minutes and will give you chance to take in some more of the coastal scenary.
Feel free to pose for photos on the rock columns, but be aware that waves will be splashing up onto the rocks, meaning that you can be soaked by the sudden swells and waves, or you can lose your footing on the slippery rocks.
There is a gift shop, selling the typical tourist wear of causeway and themed stationery, kitchenware, etc, and there is also an Information centre and Bureau du Change to Change currency.Please be aware that The Giants Causeway Visitor centre Does NOT accept Euros only Pounds Sterling if you need pounds change your foriegn currency into pounds at the exchange bureau
Check out Thyme&Co in Ballycastle just up the Road. Great for Frys and Lunches.
There is the Bushmill's Inn (http://www.bushmillsinn.com/) which is very nice and strongly Irish themed. It is about 10 minutes drive from the causeway. Given Northern Irelands relatively small size, you may feel that you want to stay nearer to or in a city such as Belfast or Derry/Londonderry and then drive to the Causeway from there. This would give you a much wider choice of accommodation.
Carrick-a-rede Rope Bridge is near by. Fun and (despite what your instincts may tell you) perfectly safe. It costs between £4 and £6, with family tickets available. A chance to walk around some more coastal countryside and walk to the Island across the rope bridge.