Earlier than 1847, Irosin was a far inland valley unspoiled by human settlers and covered by vast forest expanse teeming with wild animals. Irosin was among the last places in the Philippines to be populated. Most of the natives during the early 19th century lived along the coastal areas where it was more accessible to commercial, travel and communication activities at the time. The place have been an undisturbed sanctuary for wildlife until settlers from Bulusan in search of wild quarry and rattan came. These pioneers, using the slash-and-burn method, cleared a settlement beside a river which gave birth to a clearing called Hin-ay, an Albayanon word which denotes the arrangement of the abaca yarn or tupos into a zigzagging pattern (hinan-ay) in preparation for its actual weaving. Accordingly, the hin-ay or the pattern was the image drawn to connote the waving rivers crisscrossing from the mountains down the valley. In yet another account, the word hin-ay was purported to have been derived from the term an-hay which means gradually ascending. The original settlement which is now called San Agustin is located east of the town proper at a higher elevation, thus the gradual ascent from the lower villages to the then barrio center of San Agustin.
It was in 1847 when Hin-ay was declared a barrio of Bulusan. Deeply influenced by Catholicism brought by the Spanish friars the native of Hin-ay constructed a makeshift chapel and dedicated it to their patron, Saint Michael of Archangel. As years passed, the original clearing has grown into a bustling community and eventually the native Irosanons requested the church authorities to bestow upon San Miguel a status independent and separate from that of Bulusan. The petition was granted and Holy Masses on Sundays and holidays of obligations were held regularly.
In 1876, the Parish of San Miguel was officially recognized with Father Mariano Miranda as its parish priest. Town fiesta, in honor of San Miguel, was celebrated every 29 September and consequently, the name Hin-ay was changed to San Miguel. On the 12 December 1879, through a Royal Decree executed by the Governor General Domingo Moriones y Murillo, San Miguel became a duly constituted town and in 1880, it flourished as an independent political unit under the Province of Albay when Sorsogon, until the 10 October 1894 was not yet separated from its mother province.
It was during the incumbency of Father Esteban Rivera in 1887 when the name San Miguel was again changed to Irosin. The word was derived from the local term iros which means to cut-off a part. The word was traced to have come from an old tailor's term synonymous with tabas as in "irosi an hiniro" or cut a part of the cloth. Iros was the image most probably drawn to connote the gush or flow of floodwaters eroding riverbanks and cutting through lands to eventually form another river or river route.
