Lifespan: 15xx to Keichō 18 (1613)
Other Names: Magokurō, Shchirōzaemon
Title: Deputy Governor of Kazusa, 式部左衛門
Clan: Ryūzōji (Isahaya-Nabeshima family)
Lord: Ryūzōji Takanobu → Ryūzōji Masaie → Ryūzōji Takafusa → Nabeshima Katsushige
Father: Ryūzōji Akikane
Children: Isahaya Kiyonao, Isahaya Naonori
Ryūzōji Ieharu served as a bushō from the Azuchi-Momoyama to early Edo periods. Ieharu was a senior retainer of the Hizen-Saga domain. Ieharu served as the first lord of the Isahaya-Nabeshima family, one of the Four Families of the Ryūzōji – four families from the Ryūzōji clan folded into the governance structure of the Saga domain by the Nabeshima clan in the Edo period.
Ieharu was born as the son of Ryūzōji Akikane, a bushi and retainer of the Ryūzōji clan.
In 1580, during a siege of Yanagawa Castle in Chikugo Province, Ieharu led the third division in battle. After the death of Ryūzōji Takanobu in 1584, when Nabeshima Naoshige moved to Saga Castle, Ieharu replaced him as the lord of Yanagawa Castle. During a subsequent period of instability in the Ryūzōji clan, Ōtomo Yoshishige (Sōrin) dispatched Tachibana Dōsetsu and Takahashi Jōun to Yanagawa Castle but Ieharu defended against them. Meanwhile, Ieharu set some distance between himself and Takanobu who, in 1581, murdered Kamachi Shigenami (a daimyō and the lineal heir of Kamachi Akimori to whom the Ryūzōji were deeply indebted) and the Kamachi family. He also engaged Kamachi Sadahisa from the Kamachi clan as a retainer.
In the allocation of territory accompanying the Pacification of Kyūshū carried out in the sixth month of 1587 by Toyotomi Hideyoshi, Ryūzōji Masaie (Takanobu’s successor) received official recognition of his rights to seven districts in Hizen but his landholdings in Chikugo were seized. Consequently, Yanagawa Castle was given to Miike Shigezane, a yoriki, or security officer, of Tachibana Muneshige so Ieharu lost this base. While Hideyoshi headed toward Ōsaka, Ieharu pursued him and Akamagaseki in Shimonseki, pleaded his case, whereupon it was decided that he would be granted the landholdings of Saigō Nobunao, the landlord of Isahaya in the Takaki District of Hizen (owing to the failure by Nobunao to serve in the campaign). In the eighth month, Ieharu led 2,500 mounted soldiers and, together with 1,000 mounted soldiers provided as reinforcements by Masaie, attacked Isabaya. This army chased away Nobunao from Takagi Castle, pursuing him to Shimabara and then seized the land. This area continued to be called Isahaya although thereafter written with different characters.
Owing to discord between the Nabeshima clan who held the real authority in the territory of the Ryūzōji and Ryūzōji Takafusa (Takanobu’s grandson), Ieharu was summoned by the Edo bakufu so, together with Ryūzōji Nobuchika and Ryūzōji Naganobu, he traveled to Kyōto and acknowledged the transfer of authority to the Nabeshima clan. Thereafter, after his second son and lineal heir, Naonori, inherited the headship of the clan, the Ryūzōji surname was hesitatingly changed to Isahaya. Accordingly, Naonori was known as Isahaya Naonori and the family survived as one of the Four Families of the Ryūzōji until the end of the Edo period. Ieharu did not get along well with his eldest son, Isahaya Kiyonao, who underwent the rites of tonsure in Takeo, changed his name to Takaya Iritsubo, and served Mōri Hidenari of the Mōri clan while his grandchildren served as retainers of the Chōshū domain.