The Church of Hagia Eirene (Greek Αγία Ειρήνη, “Divine Peace”) is a Byzantine church located within in the first courtyard of Topkapı Palace in modern Istanbul. After Hagia Eirene built by Constantine burned down, it was rebuilt by Justinian in the 6th century and extensively rebuilt by the Iconoclast emperor Constantine V in the 8th century, when its apse was decorated by a simple mosaic cross. After functioning as an arsenal or armory for centuries in the Ottoman era, it became the first museum of the Ottoman Empire in the 19th century.

The first church dedicated to Hagia Eirene is generally attributed to Constantine (306-337). There are other sources, though, suggest that there was already a temple or small church here that Constantine (or his son Constantius II) rebuilt or expanded. Hagia Eirene, which was also known as the “Old Church” (Αρχαία Εκκλησία), served as the cathedral of Constantinople until the inauguration of Hagia Sophia in 360. Following the ordination of Paul the Confessor as the bishop of Constantinople in Hagia Eirene around 337, there was a series of conflicts between Orthodox and Arian Christians, eventually resulting in the death of 3150 people. Hagia Eirene was within the same enclosure of Hagia Sophia following its construction, and was part of the episcopal complex. It is possible that the First Ecumenical Council of Constantinople in 381 was held at Hagia Eirene. The church probably temporarily functioned as the cathedral at the beginning of the 5th century, following the destruction of Hagia Sophia in 404.  The relics of St. John Chrysostom, brought to Constantinople from Asia Minor in 438, were first housed in Hagia Eirene before being transferred to the Holy Apostles.

Hagia Eirene was rebuilt by Justinian (527–565) in a form of a domed basilica after Constantine’s Hagia Eirene burned down in the Nika Revolt in 532. The church then was part of a major construction project undertaken by Justinian that included more than 30 other churches, such as Hagia Sophia and Sergius and Bacchus. The adjacent structure to the south of Hagia Eirene is perhaps the ruins of the Hospital of Sampson, which Justinian had rebuilt around the same time. Hagia Eirene was repaired after a fire ravaged the surrounding neighborhood, destroyed the church’s atrium and damaged its narthex in 564. The church was severely damaged by the earthquake of 740 and probably reconstructed by the Iconoclastic emperor Constantine V (740-775), during which time the famous Iconoclastic cross was added. It is possible that, in restoring this church, Constantine V was associating himself with Constantine, the original founder of the church, as well as Justinian and Theodora, whose monograms appear on the capitals in the aisles. In 859, a council held in Hagia Eirene deposed of Photius and declared Ignatius to be the legitimate Patriarch of Constantinople. There are only a limited number of references to Hagia Eirene through the rest of the Byzantine era, such as references to it being a possession of Hagia Sophia following the Byzantine reconquest of the city from the Latins in 1261.

Hagia Eirene was apparently briefly used by Christians working in the court of Mehmet II following the Ottoman conquest of the city in 1453. A wall was build that separated it from Hagia Sophia, when Topkapı Palace was expanded to include Hagia Eirene. As the building was located in the courtyard of the palace near the barracks of the Janissaries, it served as an arsenal (known as İç Cebehane, “Inner Arsenal”). In 1726, during the reign of Ahmed III, it functioned as an armory (known as Dar-ül Esliha “House of Weapons”). From 1846 to 1874, it housed the Museum of Antiquities and subsequently as the Military Museum. In 1916/17, several porphyry sarcophagi were transported from here to the Museum of Antiquities (now Istanbul Archaeological Museums) and finally in 1946, the rest of the collection (save the limited number of artifacts in the atrium) were moved to allow the building to be researched. The structure has been affiliated with Hagia Sophia Museum since 1939. After serving primarily as a concert hall for many years, it was opened to the public as a museum in 2014.
The plan of Hagia Eirene is around 57 x 32 meters (not including the atrium), and the diameter of the main dome is around 16 meters. Its masonry mainly consists of thin red bricks and mortar. The architectural features of the church are complicated by the fact that it was restored several times. While there is some debate about the details, it is generally held that the lower sections of the church date to the reign of Justinian, while the superstructure of the church was restored by Constantine V after the earthquake of 740 (now dated by dendrochronology after 753). In addition, the remains of several structures (including a round structure) are located north and east of the church, and seem to date to the 6th or 8th century. During the Ottoman era, the columns supporting the galleries were replaced by smaller columns supported by blocks and the atrium was altered. The Ottomans also built a new gallery as well as a new main entrance.

Justinian’s Hagia Eirene was probably a domed basilica that consisted of a dome resting on pendentives raised above an elongated nave, with a narthex to the west, an apse to the east, two aisles separated from a nave by columns at ground level and vaulted galleries on three sides. The walls of Justinian’s church were probably covered by marble revetment, while it seems that the mosaics in the narthex also date to this period. There are also several capitals in the aisles bearing the monograms of Justinian and Theodora. The reconstruction of Constantine V followed the basic plan and scale of Justinian’s church, however several key changes were made to the superstructure, including adding a dome over the western bay. The most significant improvement is the transverse barrel vaults over the galleries in order to better supporting the dome on pendentives. The rebuilding of the barrel vaults of the gallery resulted in the church becoming a cross-domed basilica. Traces of vegetation set among dense geometric patterns centered on jeweled crosses can be seen in the south aisle. Chancel Proconnesian marble panels, once used as the chancel screen dividing the clerical bema from the worshippers in the naos, are now embedded in the floor under the columns.  One of these panels includes the monogram of Constantine V.

The mosaic decoration in the apse is arguably the most noteworthy feature of Hagia Eirene as it is a rare example of Iconoclastic art. Proponents of Iconoclasm, including Constantine V, rejected the use of figural imagery in religious art, substituting figures with symbols. Instead of the depictions of the Theotokos and Christ Child or Christ Pantokrator, as would become a dominant form of Byzantine church decoration, Constantine V commissioned a non-figural decorative program that focused on veneration of the True Cross. The mosaic cross in the apse consists of a stark black cross with flared ends on a golden mosaic background. It stands on three-stepped platform next to bans of greenish mosaics. The apse is framed mosaics with geometric patterns and foliage. There are also two mosaic inscriptions in the triumphal arch (from the Psalms and Amos) that were significantly altered (with the use of paint) during a restoration.