I hope I'm not adding any fuel to fires around here...
There have been plenty of views on the (Balkan) Dardanians. Here's Papazoglou's view in 'The Central Balkan Tribes in pre-Roman Tribes', after summarizing the various views expressed before her and a lengthy look into the ancient characterization of the Dardanians (sometimes as Illyrians, others as distinct from them but she considers that to have been in a 'political' rather than 'ethnic/linguistic' sense) and the few onomastic and toponymic material that have come down to us:
The ancient authorities counted the Dardanians among the Illyrians. Judging by linguistic remains, the Illyrian element played a rather decisive part in forming the Dardanian ethnos. Considerable masses of a Thracian population were included in the Dardanian community so that in historical times the eastern part of Dardania [ed: roughly the Scupi-Naissus line] had a markedly Thracian character. Political developments contributed to the differentiation of the Dardanians as a separate people
Ditto with the ancient Epirotes. Here are some recent views on the ancient Epirotes spanning the breadth of possibilities but that generally agree with the predominant Northwest Greek character of the onomastic and epigraphic remains.
N.G.L. Hammond in The Oxford Classical Dictionary 2nd and 3rd Ed. respectively, under Epirus:
...fourteen Epirote tribes, probably of Dorian and Illyrian stocks.
...fourteen Epirote tribes, speakers of a strong West-greek dialect.
William Bowden in Wiley's The Ancient Encyclopedia of Ancient World History, under Epirus:
Despite Thucydides describing them as barbaroi (2.68.5), it seems certain that the language spoken by these tribes was a dialect of Greek (an issue that has caused considerable dispute in relation to the modern geopolitics of the region).
Daniel Strauch in Brill's New Pauly, under Epirus:
Ancient authors saw the inhabitants of E. as bárbaroi...However, there was an early Hellenization of the elite, the inhabitants of the coastal towns, and also the sanctuaries. The original language is unknown; rare early written evidence from Dodona shows Corinthian letters with local (?) deviations, the earliest (longer) inscriptions of c. 380 BC are written in a north-west Greek dialect.
In general, it seems that the Dardanians were an Illyrian-speaking population in whose area in the Easternmost parts the Thracian element became predominant over time, while the Epirotes, whatever their 'original' language, seem to have adopted Northwest Greek at least by the early 4th century BC and visited by
theorodokoi starting around the same period. It's true that they didn't fit in quite well in the "Greek proper" concept of ethnicity (see Malkin and Hall's work on that) but we don't have an emic view of the Illyrians and the Dardanians in the first place. Of course perfect linguistic and ethnic 'purity' should probably not be sought in all those borderland regions and the area of south Illyria with cities like Dimale and Byllis shows that well.