Irāvatī(Iravati) is a famous holy river of the Vedic period. In the Vedic era itself, Irāvatī had already been marked as an alternative name of river Paruṣṇ̣ī (Parushni). In Nadī-sūkta(Nadi-sukta; Hymn dedicated to rivers) of Ṛgveda (Rigveda), river Paruṣṇ̣ī is praised along with the other ancient rivers like Gaṅgā (Ganga), Yamunā (Yamuna), Sarasvatī (Sarasvarti), Śutudrī(Sutudri) in the same mantra   ─
imaṃ me gange yamune sarasvati
śutudrī stomaṁ sacatā paruṣṇyā.

Īrāvatī is the river Paruṣṇī of the Vedic age. Etymologically, the term Irāvatī can not possibly derive from Paruṣṇī . But Irāvatī can of course derive from Airāvatī (Airavati). It is not even impossible that with the removal of the initial ‘I’ , Irāvatī became Rāvatī, and through an extreme instance of apabhraṁśa (apabhramsa; a broken language distorted from Sanskrit, Pāli and Prākṛta), it might have been ‘Rābhī’ (Rabhi). Though the etymological derivation of Irāvatī from Paruṣṇī cannot be proved, the evidence that Paruṣṇī was called Irāvatī in the Vedic times, can be found in the lexicon-text called Nirukta dated BCE. Yāska had written in Nirukta, that Irāvati was called Paruṣṇī —
irāvatī paruṣṇītyāhuḥ

We have not found the name Irāvatī in the Nadi-sūkta of Ṛgveda. However, since Yāska had said that Irāvatī was Paruṣṇi itself, and since a learned person like Maxmuller has kept Paruṣṇī, Irāvatī and Rābhī as serial-terms in the same bracket, it is understood that all adjectives and information about Paruṣṇi are also applicable to Irāvatī.

Patañjali, the author of Mahābhāṣya (Mahabhashya), dated  circa 150 BCE, while writing commentary on a sūtra (sutra; formula) by Paṇini [2.1.20], has mentioned the river Irāvati, in the context of describing the land watered by the said river —
dvīrāvatīko deśaḥ. trīrāvatīko deśaḥ.
In one mantra of Ṛgveda, Paruṣṇī has been called ‘Mahānadī’ (Mahanadi, Great River)–
satyamittvā mahenadi paruṣṇyavadediṇśam.