The Illyrians lived there. They are a very poorly-understood group who left no writing behind, but we understand they spoke an Indo-European language. Their culture was typical Indo-European tribal in structure and, from the Bronze Age to the Early Iron Age, developed a mixed economy of herding and small-scale agriculture. There were several population centers in the Iron Age (ca.700 to 100 BCE), including a large one at Glasinic. As far as their material culture, they seem to be fringe participants in the larger "urnfield culture" of the early Iron Age in central Europe, and later seem to have material contacts with the widespread Celtic culture called La Tene. There is a lot of argument about the origins and organization of the Illyrians, and, unfortunately, the Romans paid them very limited attention. There is not, consequently, a lot of information. They are very similar to their northern neighbors, the Celts (Roman "Gauls") in just about every way, though they were probably technically separate from Celts linguistically.

It is possible that Illyria, as it was probably the case for Iberia or Ligustikè was rather a geographical space broadly defined by its contact with Greeks than an ensemble of peoples that necessarily thought themselves as Illyrians.

While an Illyrian language family is often hypothesised but its existence is arguably debatable : apart from onomastic or toponymic evidence and *maybe* inscriptions^(1) (whom translations more or less importantly varies depending the researcher proposing a translation) found in Italy where the Messapian people lived who might have been Illyrian. That's not much : it doesn't mean that a separate Illyrian language didn't existed, at least Greeks might have thought it did, but it remain and hypothesis and when languages as Italian Venetic are tentatively identified they're generally classed together with other families (in this case, Italic).
A relation between the Illyric languages and Gaetic family had been proposed, but it's tentative, and while plausible, there's nothing really proving that Albanian is the modern offspring of "Illyrian". That said, new archaeological discoveries and the possibility of additions to an Illyrian corpus in the future shouldn't be dismissed.

There's barely anything to say about a distinct Illyrian people in the first Iron Age, elements of identification being essentially Greek, who identified a mythic ancestor named Illyrios which founded a people named Illyrians that were kin to Celts and Galatians, Illyrios having sons that themselves founded smaller peoples as Pannoni. This genealogical history was commonly used by Greeks to explain why some groups shared features and ties, but as well why they were more or less "open" to the Hellenic civilization. Giving the names involved, Illyrians peoples were supposed to live between the Adriatic and the Danube, roughly the southern half of former Yugoslavia (from modern Croatian shore to modern Albanian shore, roughly). It's not impossible that you had a blurry difference between Italic and "Illyrians" on the Dalmatian shore.
The lack of a relatively homogeneous material culture (except maybe the Glasinac culture, which doesn't cover all of the Illyrian shore) doesn't mean anything (people could share language or even institutions without partaking to a same archaeological horizon) but it makes identifying a distinct "Illyrianity" hard to say the least.
Eventually, Illyrians are best identified trough their relations with Greeks, with the constitution of coastal petty-kingdoms; and it's possibly the best perspective we could have as to the distinction of a western Balkanic people, defining itself with his relations to his southern neighbours. In this sense, Illyrians didn't necessarily pre-existed Greek contacts, but might have been individualised and organised due to these (not unlike Celts in southern Gaul). The Glasinac culture would be then representative of this cultural and social development of these people, later called by Romans "true Illyrians" out of a ill-defined Thraco-Geto-Dacian ensemble, with which "true" Illyrians seems to have kept relations with such as with Paeoni.

These petty-kingdoms seems to have reinforced themselves trough trade, mercenariate (maybe as far as the pre-Archaic Age) and piracy, neighbouring Hellenic or Hellenized entities as Epirote kingdoms or Macedonia; forging dominance over the Adriatic region from time to time, especially Dardanians and Ardians, until Romans had enough and saw an opportunity to use the coast as a buffer/watch against Macedonians.

It's probably a bit vain to search a definite ethnicity to northern ("False") Illyrians peoples, but bearing features of Latenian material culture from the IIIrd century onward could indicate a certain degree of Celticization trough migrations and/or conquest., especially along the Danube. Some people might have been made of various origins although Celticized (Iapodes), some more Celtic in appearance and definition (Scordisci, although probably mixed too). It's not easy, and it might be impossible, to make a clear difference between the indigenous mix of "true" Illyrians, probably kin people of the hinterland, and Celts. It's probably why the aforementioned tale linked the ancestor of Illyrians with Galatians and Celts.

- Les Celtes, histoire et dictionnaire; Venceslas Kruta; 2000

- Les Celtes orientaux et la formation des Scordisques : aspects archéologique, numismatique et chronologique in Celtes au IIIè siècle avant J.-C.; Petar Popovic; 1993

- Illyrian Language in Encyclopedia of Indo-European Culture; James Mallory, D.Q. Adams; 1997

- Illyricum in Roman Politics, 229 BC–AD 68; Danijel Dzino; 2010

- The Illyrians; John Wilkes; 1992

^(1) Klohizisthotoriamartapidovasteibasta Veinzanaranaindarantoavasti .
Where Basta might be a place (it's the most agreed on translation), and it might be question of a gift of a land. Make of that what you will.