Hirai Tsuneharu served as a bushō during the Sengoku and Azuchi-Momoyama periods.
Tsuneharu was the head of the Hirai clan, a gōzoku, or powerful clan, in Hizen Province. He held the titles of Senior Assistant Minister of Popular Affairs and Provisional High Steward. In historical accounts he was praised as a valorous warrior without equal in his era.
The Hizen-Hirai clan was a branch of the Mutō clan. Arima Haruzumi of the Hizen-Arima clan wed his daughter to Tsuneharu so that Tsuenharu became his son-in-law, granting him the Kishima District and assigned him to protect this territory. In 1541, Taku Munetoki betrayed Shōni Fuyuhisa in favor of Arima Haruzumi and when an emboldened Haruzumi dispatched troops in an effort to destroy Chiba Yoshitane, in addition to the Shōni and Ryūzōji clans, Tsuneharu served in the vanguard. At this time, however, the Chiba clan which was divided between east and west branches formed a cooperative alliance so the Arima forces withdrew before the situation escalated further.
In 1562, the Ōtomo clan sounded out the Arima clan regarding cooperation to revive the Shōni clan which had been decimated in the previous year by Ryūzōji Takamoto. Arima Haruzumi agreed and raised an army to eliminate Takanobu; however, given that the Arima had been intercepted by the Ryūzōji forces, Takanobu came on the attack, so the Arima had to request support from Tsuneharu and Gotō Takaakira at Tsukazaki Castle. At this time, after losing his base to the Ryūzōji, Taku Munetoshi sought support from Tsuneharu but then Takanobu ordered an attack on Tsuneharu led by Nōtomi Nobukage and Nabeshima Nobumasa (later known as Nabeshima Naoshige) with an army of 2,000 soldiers. Tsuneharu departed from his base at Suko Castle (also known as Taka or Takaoka Castle) to intercept the approaching army, and after decimating the forces, engaged in a pursuit while they fled. In the eighth month of 1563, after Gotō Takaakira marched into Tsuneharu’s territory, once again, Tsuneharu repelled the invading forces.
In the second month of 1564, when Ryūzōji Takanobu marched forward to challenge Tsuneharu again, Tsuneharu’s forces attempted to intercept them but were overpowered and forced to retreat. Nevertheless, Suko Castle was a stronghold and, knowing that an assault would be difficult, proposed a reconciliation. Tsuneharu accepted the proposal and a settlement was reached on the condition that he arranged for his younger brother, Hirai Naohide, to wed the adopted daughter of Takanobu (the natural daughter of Ryūzōji Nobuzumi) and promised to destroy the large moat and embankments at Suko Castle.
In the seventh month of 1574, acting on the perception that Tsuneharu would betray him, Takanobu took action to subdue Tsuneharu. Tsuneharu, together with his younger brother, Naohide, charged out and, for several days, gained the upper hand against Takanobu. However, after their position gradually began to slip, Tsuneharu and Naohide retreated to Suko Castle. In a bid to topple the fortress of Suko Castle, Nabeshima Nobumasa persuaded Naohide and almost all of the retainers of the Hirai clan to betray Tsuneharu in favor of the Ryūzōji. Tsuneharu was forced to flee to Yoshida in the Fujitsu District, but, in the tenth month, Tsuneharu and his uncle, 新宗吟, attacked and destroyed Naohide and achieved a return to Suko Castle. Upon hearing this new, Takanobu sent an army the following month to destroy Tsuneharu. After fighting for over a month, Tsuneharu lost and Suko Castle toppled.
According to one military chronicle, Tsuneharu either committed seppuku or was killed by a stray arrow while attempting to flee the castle. According to another account, Tsuneharu attempted to commit seppuku but was stopped by his retainers and, after fleeing the castle, sought help from Gotō Takaakira, and, two years later, entered Ueno Castle (also known as Uwado Castle), but the details of his whereabouts thereafter are unknown.