The grandson's preface is the anchor. He says he came to Egypt in the thirty-eighth year of King Euergetes and there translated his grandfather's Hebrew book for Greek-speaking Jews, apologising for what translation costs a text. Euergetes is Ptolemy VIII, whose thirty-eighth year is 132 BCE, which places the grandfather's composition roughly two generations earlier, around 180 BCE, before the Maccabean crisis, of which the book shows no knowledge. Almost no other Jewish book of the period can be dated so tightly.
The Hebrew original was then lost for the better part of two thousand years, and Sirach was known in Greek, Latin and Syriac. In 1896 fragments of a Hebrew Sirach were identified among material from the Cairo Geniza, the discarded-manuscript store of the Ben Ezra synagogue in Fustat. They were medieval copies, eleventh and twelfth century, but they were Hebrew. Further leaves surfaced over the following decades, and between them the Geniza manuscripts supply roughly two-thirds of the book.
Ancient confirmation followed. Cave 2 at Qumran yielded a small fragment, and the great Psalms scroll from Cave 11 contains a poem also found at the end of Sirach. Then excavation at Masada in the 1960s produced a leather scroll of the first century BCE carrying a substantial run of chapters, and it agrees closely with the medieval Geniza text. That agreement is the decisive result: it showed that the Geniza copies transmit the ancient Hebrew rather than a medieval retranslation out of Greek or Syriac, and it made the Hebrew usable for reconstructing what Ben Sira actually wrote.