Straight from them to you:
Mostly hailing from the mountainous areas of what is now the Balkans, the Illyrians were a large and diverse group analogous with the Celts and Germans. The Illyrians lived to the West of Macedon and were a traditional foe for the Macedonians. Illyrian forces excelled in raiding and would do so either by land or sea, enslaving great numbers.
Following Philip’s subjugation of the Illyrians tribes they would be found in Alexanders armies as light infantry. Wearing sleeveless, unbelted tunics, often bearing vertical stripe patterns, the Illyrians are easy to distinguish from other troops.
These Illyrian infantry models can be used to represent light infantry or peltasts from the many Illyrian tribes or in the employ of Alexander the Great.
Those or you playing Hail Caesar will find an army list for the Illyrians in their own right as well as entries for them in other armies within the pages our Hail Caesar Army Lists: Biblical and Classical supplement.