Folk clubs are usually run by local music enthusiasts. They tend to be run in rooms in hotels or pubs and usually licensed for drink. Don't be put off by the word "club" - you do not need to be a member and visitors are always welcome. They are less formal than a concert in a theatre, you will be much closer to the performers and be able to talk to performers about their music. Although tending to be informal, you will be expected to be quiet during the performances - it is not background music.
When clubs have a guest, these will be artists who have been booked usually to do two 30-40 minute performances supported by other local performers perhaps only contributing one song or two. If you are a performer yourself and make yourself known to the organiser, you may well be welcomed to "take the stage".
Clubs often have nights referred to as "singers nights". For these all the performances come from people who turn up to perform and there are no professional guest spots. These nights tend to be even less formal although you will still be expected to respect the courtesy of listening rather than talking during performances. You will be welcomed to take part but you don't have to. Listeners are welcome.
Use the links below for club lists and links