That sounds like an amazing plan, Anja! 6 days is plenty of time to experience Halloween and more.
Derry~Londonderry is known to have a unique atmosphere during Halloween, but that's the case in many other cities and towns. Dublin also has many things going on for Halloween and day trips options by
public transport, such as the nearby
coastal towns and the Wicklow Mountains. These are two great places for
hiking. If you'd like to stay in
Belfast for a few days and explore by public transport, that's also an option.
Furthermore, you'll find more info on public transport
here, which is really convenient all over the island.
Renting a car is also an option if you'd like to go on a road trip and maybe visit places that are more remote.
Finally, we have
plenty of distilleries all over the island. Dublin has Jameson and Teeling, while Bushmills is quite famous along the Causeway Coast (between Belfast and Derry~Londonderry)
I hope this helps!
Een fijne avond gewenst,
Antoine