Hi folks hope you are all well I am new in this forum, I am from Northern Ireland so feel free to ask Northern Ireland related questions ....I did want to answer this question, you can go to the giants causeway via public transport it easy and quick and cheaper than a tour which will cost £50 or more (fifty UK pounds that is) here is how to do it via public transport from Belfast
would recommend you DONT do a prebooked tour as you don't get much time at each location, ok given using public transport takes effort and time but at least you can take your time etc, its very easy to do as i said before, just take the train from Great Victoria Street Train station (entrance is beside europa hotel marked "Great Northern Mall" follow the corridor down to the bus station and turn left then right into the Train station), Buy a £17.50 Zone 4 I-link card this gives you unlimited travel on all Bus and trains anywhere in Northern Ireland for 1 day travel (you can get for a week and a month if you are here that long lol), Jump on the train to Derry/Londonderry @ Great Victoria Street Train station, change in Coleraine after 1hr 25 min of a journey on the train to COLERAINE the bus and train station in Coleraine is under one roof go in and catch the Ulsterbus 402 service to Ballycastle this drops you off yards from the Giants causeway visitor centre, do not go into it as its a tourist trap, go down to the stones via the tunnel to the right of the visitor centre, (when in the bus station/train station walk to the end of the station the bus departures go from the far end of the building there is departures board there listing the departures)
public transport in Northern Ireland is operated by Translink (www.translink.co.uk)
remember you need UK Pounds for Northern Ireland not euros