The Hagia Irene, located in the outer courtyard of the Topkapi Palace, is the oldest church of the Eastern Roman Empire (Byzantine). It was also the second largest church in Istanbul after Hagia Sophia. Hagia Irene, originally meaning "sacred peace" (Hagia Eirene), holds a special meaning as the birthplace of Turkish museology. This typical Byzantine structure, built on top of an older temple out of wood in 330, was destroyed during the Nika Revolt in 532, along with Hagia Sophia. Both churches were later rebuilt under the reign of Emperor Justinianus.
Hagia Eirene comprises three sections: naos, narthex, and atrium, making it the sole surviving example of a Byzantine period church with a standing atrium. Originally constructed as a basilica with three naves during the reign of Konstantinos the Great (306–337), it underwent repair by Leo III (717–741) and his son Constantine Copronymus (741–775) following the devastation caused by the great earthquake of 740. The upper structure was entirely refurbished during this restoration, resulting in the building's current appearance as a domed church with a basilical plan.
Although the structure may have been lavishly adorned with figurative ornaments during the reign of Justinianus (527–565), this was completely altered during the repairs of Leo III and Constantine Copronymus, both of whom were emperors of the Iconoclastic era. The cross depiction visible today on its half dome was created during that period.
After the fall of Constantinople, the church was not converted into a mosque but used as a loot and gun depot known as the Harbiye Warehouse. It was later repurposed as a weapons depot. In the 19th century, it was named the "Harbiye Ambari" (war academy depot) and later became the country's first official museum, known as Muze-i Humayun (Imperial Museum). It also served as the first Military Museum between 1908 and 1940.