The short of it is that Prof. McCone theorizes that the Gundestrop Cauldron is associated with fiana like warriors of Gaul.  All based on the images on three of the inner panels of the cauldron.  I will paraphrase his work from his article “The Celtic and Indo-European origins of the fian” found in The Gaelic Finn Tradition published by Four Courts Press.

Panel 1 – This is the well known ‘Cernunos’ panel, the main figure with Gaulish parallels (see “Keltische Religion (1961 Pp106-7))  with horns on his head and the “hatching” of the clothing worn is indicative of fur.  A deer stands on the left and a wolf on the right, with the horns associating him with the deer and the fur possibly associating him with the wolf.  The implication being that he has attributes of both animals.  Above and below the figure are a goat and lion, which would have the same cultural attributes as the deer and wolf near Eastern or Balkan context.  Keep in mind that the cauldron most likely comes from Thrace, which was a mix of Eastern and Gaulish peoples.

Prof. McCone believes that the figure depicted is a patron of the *koryos the outcast warrior bands of Indo-European cultures such as the Fiana in Ireland.

Panel 2 – The second panel has hunters accompanied by hounds or wolves attacking large oxen.  Above each stands a spotted cat like figure that could be a leopard. The prey our doubtless to be aurochs with the leopard being another Balkan addition.  If the dog figures are hounds then they would be helping with the hunt, if wolves they could represent the ‘mascot’ of the hunters, who garments resemble that which is worn by the “Cernunos’ figure on the first panel.  The two outer men are wearing only britches and cap with the more seasoned leader wearing a garment covering his upper body.  With the stylized leaves, also found on the first panel, this could be a *koyros hunting party in the woods.

Panel 3 –  The third panel depicts a ritual, where a group of foot soldiers move towards a cauldron are dunked into the cauldron and come out as horseback warriors.  An initiation or cleansing from one status to another.  The bottom six soldiers are armed with spear and shield and are again wearing clothing as depicted on the ‘Cernunos’ panel.  At the front of the line of soldiers is a wolf facing the oncoming soldiers, with musicians at the end of the line.  The wolf is presumably the mascot and the young men are the ‘young wolves’ preparing for initiation back with the musicians also being ‘young wolves’ not yet ready for the transition but acting as assistants to the ritual.

I will quote McCone’s summary:

Whatever about some of the details, the crucial point here is that the three scenes from the Gundestrup Cauldron, just discussed, present clear evidence for a sequentially regulated Gallo-Thracian opposition between, on the one hand, spear and shield, and, on the other, a class of mounted warriors, progress from the former to the latter set being marked by a baptismal rite of passage. 

For my own part.  I have to wonder if the cauldron was actually used for such rituals.  Now that would be an amazing thing.

Further reading:

de Vries, J. Keltische Religion (1961), pp 104-07