Irobe Masanaga served as a bushō during the Sengoku period. He was a kokujin, or provincial landowner, in Echigo Province. Masanaga served as the lord of Hirabayashi Castle in the Iwafune District of Echigo.
The Eishō Conflict was a series of battles across the Kantō and Hokuriku regions that occurred in Eishō era (1504 to 1521) during the early part of the Sengoku period. One of the series of events comprising this conflict is as follows:
In the ninth month of 1506, Nagao Yoshikage, the deputy military governor of Echigo, died in battle in Etchū Province. His eldest son, Nagao Tamekage, became the next head of the Nagao clan and deputy military governor of Echigo. In the eighth month of 1507, Tamekage backed Uesugi Sadazane in a sudden attack against Uesugi Fusayoshi, the military governor of Echigo. Masanaga and many other members of the Agakita Group allied with Fusayoshi who relied upon Uesugi Akisada, the deputy shōgun of the Kantō, in an attempt to flee to the Kantō, but Tamekage cornered him at the Amamizu Pass where, together with Maruyama Nobuzumi, Fusayoshi killed himself. Thereafter, Masanaga, together with Honjō Tokinaga and Takenomata Kiyotsuna, continued to resist. Tamekage’s active military operations became a burden for the kokujin of Echigo who grew dissatisfied with him.
Nakajō Fujisuke and members of the Tsukiji clan from the Agakita Group associated with Tamekage and attacked Masanaga and his allies. Masanaga holed-up in Hirabayashi Castle but, in 1508, the castle fell and Masanaga surrendered to Tamekage.