Dionysius of Halicarnassus derives them from Pelasgus himself, and they existed as Achaeans before the Hellenic Xuthus was even born.

The legend which makes Achaeus the brother of Ion, tends likewise to prove, that if the Ionians were originally Pelasgic, so also were the Achaeans.

Let us then come to Ion.
Although Ion is said to have given the name of Ionians to the Atticans, yet long before his time the Iaones were among the ancient inhabitants of the country; and Herodotus (the best authority on the subject) declares that the Ionians were Pelasgic and indigenous.
There is not sufficient reason to suppose, therefore, that they were Hellenic conquerors or Hellenic settlers.

They appear, on the contrary, to have been one of the aboriginal tribes of Attica:--a part of them proceeded into the Peloponnesus (typified under the migration thither of Xuthus), and these again returning (as typified by the arrival of Ion at Athens), in conjunction with such of their fraternity as had remained in their native settlement, became the most powerful and renowned of the several divisions of the Attic population.

Their intercourse with the Peloponnesians would lead the Ionians to establish some of the political institutions and religious rites they had become acquainted with in their migration; and thus may we most probably account for the introduction of the worship of Apollo into Attica, and for that peaceful political influence which the mythical Ion is said to have exercised over his countrymen.
At all events, we cannot trace, any distinct and satisfactory connexion between this, the most intellectual and brilliant tribe of the Grecian family, and that roving and fortunate Thessalian horde to which the Hellenes gave the general name, and of which the Dorians were the fittest representative and the most powerful section.