From the quoted passage, it seems that St Jerome is referring to Śramaṇas. These are also called Samanas in Pali. The meaning of this term is "One who labours for a higher purpose or religious purpose". These people considering themselves different from the path of Vedas and Brahmins was the reason for the separation. They opposed rituals recommended in the Vedas and emphasized self-meditation and non-ritualistic practices which do not involve violence.
A reference to them can be found in Brihadaranyaka Upanishad. The Upanishad does not differentiate them from Hinduism or Vedic path. It just emphasizes meditation and seeking self-knowledge through meditation. But "Śramana" when generally referred to means a follower of Buddhism. Due to historical events, there was a clear distinction between Brahmins and Śramaṇas. There were two main sects at the time of Buddha. They were Brahmanas (Brahmins) and Samanas.
From Samana and the sects of Buddha's time from Dhammawiki by Ven. Shravasti Dhammika
The two main religious movements at the time of the Buddha were those of the brahmans and the samanas. I have not been able to check it but I think our word shaman is related to the Pali samana.........
The samanas on the other hand, rejected the Vedas and most Brahmanical beliefs and practices and were considered unorthodox, even heretical, by the brahmans ................. Because most samanas ignored caste rules this put them on a par with low castes and outcastes in the eyes of the brahmans. They ignored social norms and expectations, they were usually celibate and in spiritual matters gave precedence to experience rather than scriptural authority. They experimented with meditation, self-mortification, yogic breathing, fasts and sensory deprivation. When, as a result of such practices, an individual had some kind of mystical experience which led him to believe he had attained enlightenment or liberation, he would attract disciples and this would lead to the founding of a sect.
Even Tirumazhishai Alwar composed a poem referring to them. In the poem, he refers to Samanas as Buddhist.
"sAkkiyam katrOm, SamaNam katrOm, SankaranAr aakkiya Aagamanool
aarAindhOM; BhaggiyathAl venkaTkariyAnai SerndOm"
I have been a follower of Jainism for long, follower of Buddhism for long, a follower of agamas written by Shiva (shaiva) too. But, due to sheer grace, I landed at the feet of TiruvengadathAN.
But later, the term is used to refer to a mendicant who has detached himself from Vedas and a follower of self-realization who is against the caste system etc., and used for other paths like Jainism too. Samanas is also a popular term to refer Jainism.
But St Jerome is talking about a classification made by Bardesanes (Latin for Bardaisan). Bardaisan got this information from an interview with a Śramaṇa who was sent to a Roman emperor. From Encounter with religious men from India from Wikipedia article on Bardaisan
For the polity of the Indians being distributed into many parts, there is one tribe among them of men divinely wise, whom the Greeks are accustomed to call Gymnosophists. But of these there are two sects, one of which the Bramins preside over, the Samanaeans the other. The race of the Bramins, however, receive divine wisdom of this kind by succession, in the same manner as the priesthood. But the Samanaeans are elected, and consist of those who wish to possess divine knowledge. And the particulars respecting them are the following, as the Babylonian Bardaisan narrates, who lived in the times of our fathers, and was familiar with those Indians who, together with Damadamis, were sent to Caesar. All the Bramins originate from one stock; for all of them are derived from one father and one mother. But the Samanaeans are not the offspring of one family, being, as we have said, collected from every nation of Indians.
From the description given by St Jerome, it looks like he is referring to Jains who are strict vegetarians and who were present in great numbers in Varanasi during the early first century. Four Jain Tirthankaras were born in Varanasi and many Jain Shrines are located around the banks of the Ganges there. It was a dominant religion in India around 1st and 2nd centuries. We can still find Jain Shrines in Uttar Pradesh.
Though they were dominant in the ancient times, the population has now declined and become a minority in many states in India.