Some time ago on a site that is probably not this one there was debate about a new book showing Roman cavalry using Boeotian helmets in (IIRC) the 1st century BC. The author/publisher cited the cavalryman's helmet in that artwork as a Boeotian helmet.
However, I came across this the other day:
http://www.romancoins.info/MilitaryEquipment-Helmet-weisenau.html
This is a Port helmet that would look very very like the DA frieze helmet back view and a Port helmet would fit being a Celtic cavalry auxiliary.
http://upload.wikimedia.org/wikipedia/commons/9/9a/Altar_Domitius_Ahenobarbus_Louvre_n3.jpg
Roy
This scene is thought to show the concluding sacrifice of a census - very much a Roman citizens' ceremony, and not a scene where you'd expect a non-citizen auxiliary to be shown. The helmet could still be Gaulish, though, even if the wearer is not - good suggestion, Roy.
Two possible issues, though. One, I don't know how early the oldest examples of this Agen/Alesia (not Port, that's the one with the neckguard) type are found: it might not be compatible with the early, 2d-century, end of the date range suggested for the Ahenobarbus relief - though given that the exact date of the relief is so uncertain, that is not a major objection - http://bmcr.brynmawr.edu/2004/2004-03-01.html (http://bmcr.brynmawr.edu/2004/2004-03-01.html).
Second, the Gaulish helmets do all seem to have the narrow ridge around the skull above the Boiotian-style brim, which the carved version lacks.
Previous Port helmets I have seen have more of a bowler hat form at the top, but , of course, you are right that they have a rim. that might just not make it onto the frieze. There is of course no reason why a Roman would not have a boeotian form aren't they shown on the Aemillius Paullus monument? but I thought it interesting that this Port helmet had folds in the brim and, of course, other Roman kit is of Celtic provenance'
roy
No - there are no Roman heads on the Paullus that survive in good enough condition to identify the helmet. I think the Domitius may be the only Roman source that clearly shows a Boiotian-style brim. One problem with reconstructions of Roman cavalry with Boiotian helmets is that they tend to show 4th-century "classic" helmets on much later cavalrymen, and I am not sure how far into the 3rd century they stayed around unevolved.
And see figure 10.2 in http://www.persee.fr/web/revues/home/prescript/article/pica_0752-5656_1996_num_3_1_2177 (http://www.persee.fr/web/revues/home/prescript/article/pica_0752-5656_1996_num_3_1_2177) for another Gallic helmet like the one you linked to, with the high conical skull but not so wide a brim.
Could the helmets of these chaps be cousins of the Ahenobarbus cavalry helmet?
http://www.livius.org/a/1/romanempire/gladiators_sIBCE_mus_munchen.JPG
Identified as 1st century BC gladiators.
Looks quite possible to me. They have the wave effect on the neck guard, they are right era and they have fairly conventional military equipment. Of course Romans at this time are wearing lots of different stuff.
Roy