Hello!
Does anybody know if it's possible to do the walk from carrick-a-rede to giant's causeway in one day, leaving Belfast in the morning and returning to Belfast at night? I'll be using public transport.
Yes you can. I imagine the walk will be held about three hours, but you can find the website of the Ulster Way more accurate and useful.
I think there are several forms of public transport. You can take a bus to Ballymena and change to a coastal city bus to Ballycastle or similar. With Larne would be much time for your best option might be to go from Belfast to Coleraine train / bus and bus to connect to the Giant's Causeway coast. You will need to check the bus schedule, but you can come back for Ballycastle - Ballymena - Belfast. Or vice versa.