Celtic nobility are used in numerous foreign armies, accompanied by their experienced personal guards.
Celtic nobility and aristocracy of regions conquered by foreign tribes or entirely foreign societies sometimes remain in place, occasionally even empowered over local rivals who had sided against the conqueror. These empowered men are of good standing, and are used as local administrators, or taken on campaign as a benefit to the local troops they lead. They are not as powerful as more free Celtic nobility, but they maintain a good degree of autonomy and authority, at least within their own estates.
The guards of these men are Celtic warriors of fair standing, equipped with good gear, such as mail or scale, swords, and good quality helmets. Since they are not the same as the well-trained and experienced soldiers of upper Celtic nobility, they are less stalwart in a fight. On the other hand, accompanying their leader, they are braver than most other Celtic warriors of their standing and their equipment allows them to efficiently fight numerous other enemies at a decent advantage.
Historically, many Celtic nobles campaigned for many non-Celtic people, as well as unrelated Celtic tribes and cultures. Caesar's campaigns in Gaul had a great many Gauls fighting on the side of the Romans, Galatian captains and chieftains fought in numerous foreign armies as their allies and British chiefs were used in Britain to such an extent that entire tribes were subdued for the Romans purely by allied native British troops, such as the Silurii ultimately being subdued by the Roman-allied Demetae. They fought for protection, treasure and their cultural motivation for glory and fame, and they would often work for any who could provide good chances for any of these.