Sorry ...
I thought EVERYONE knew.
It's one of the four treasures of the bronze age Tuath Dé (later renamed by monks to Tuatha Dé Danann).
It magically produces food. But how?
My characters are very cold and very hungry, they suspect what it is, as the same chamber has a spear, a glowing sword and mysterious stone pillar. An outer cave/chamber has ... well stuff that leaves them in little doubt who these things belonged to.
Of course perhaps all this has a guardian.
I thought it might be made of copper, being from near start of Bronze age. It could be made of riveted sheets of bronze as most cauldrons 600BC to 1800BC in Europe / British/Hibernia Isles are.
As I said, it probably looks like a regular 1700BC to 1800BC Bronze Age cauldron. That's not important.
My characters live in the "Otherworld" that Manannán Mac Lir took the Tuath Dé to, 600 years ago in their time-line. It was already inhabited by Elves, Dwarves, Dryads.
My Fair person (the human Tuath Dé boy), Elf, Dwarf and Dryad (all teenagers) are high in the northern mountains above the snow line and at the end of the Arctic. Time is slipping compared to our world. So that nearly 2,500 years have passed for us, the Tuath Dé left at start of our Iron age in Ireland.
So also in the "party" are an Irish girl and American boy from early 1970s, obviously if they find a "return" portal, perhaps more than forty years will have passed. The Irish girl is very well up on her Celtic myth, so privately she suspects already this might be trip you'd not want to come home from. Any reader familiar with Celtic Legend will suspect this from the instant she and the American are transported. Which currently is Chapter 10.
So the first 9 chapters are entirely set in Otherworld with not much references to "here"
revised Opening
http://www.sffchronicles.com/threads/551709/page-2#post-1893070
Start of Chapter 10 (was originally a separate story).
http://www.sffchronicles.com/threads/551787/#post-1897254
Incidentally the "Otherworld" isn't in Bronze or Iron age or even Mediaeval any more, without the dark ages etc and synergy of the other sentient species they are at about a Renaissance (1300 - 1798) level of technology. No electricity, but most of the rest we had then and some extra stuff. More magic