There is no real indication of what lies ahead when you first arrive. However within a very short time you are underground in a former lava tube on a guided tour which can only be described as impressive. Just make sure your camera is set for low light and capture some magnificent photos.
One word of warning, there are some very low ceilings in places and the rock is very hard, your head will lose the argument!
It is a bit of a trek with steps and slopes, so maybe not suitable for all. However, if you are steady on your feel it is well worth the journey.
Watch out for the "secret" it's quite impressive and draws a sharp intake of breath for most who see it. (Can't tell you what it is, or it wouldn't be a secret)
The cave has a fairly large car park but it does get busy. There are national park staff on duty at busy times to assist with parking and we got straight in.
The ticket office is part way down into the cave and it can get very hot queuing, but there is shelter from the sun whilst you wait for your guide after buying your ticket.
Multiple site tickets are available, we bought a 3 site ticket for €21 (September 2019)