Hagia Eirini ( Ἁγία Εἰρήνη / Aya İrini), also spelled Hagia Irene, was the first church commissioned in the city of Constantinople, by Emperor Constantine I himself in the 4th century. It served as the Ecumenical Patriarchate of Constantinople until the Megali Ekklisia, the predecessor to Hagia Sophia, was completed in 360.
One of the most important events in Christian history took place in the church from May to July 381. The First Council of Constantinople was the second ecumenical council and confirmed the Nicene Creed of 325. It was organized by Emperor Theodosius I.
After the Conquest of Constantinople in 1453, the church was decommissioned, turned into the Cebahane (Armory), and enclosed within the walls of Topkapı Sarayı. It was used three times as a weapons museum: 1726 to 1744 during the reign of Ahmet III; as the Museum of Early Munitions and Artifacts from 1846 for an unspecified number of years; and as the Military Museum from 1908 to 1978. After 1978, the building was converted into a concert hall. It’s one of the very few Byzantine churches never converted into a mosque by the Ottoman conquerors.
When I was living in Istanbul, the only way to gain access was to attend a concert or with special permission, but since 2014, Hagia Eirini has been open as a museum. It’s possible to visit for 20TL or with a Museum Pass İstanbul daily except Tuesdays. Hagia Eirini is part of Istanbul’s UNESCO World Heritage listing.
I was able to visit for a special exhibition and saw the interior of the church and the courtyard. The church is very plain compared to other Byzantine churches such as Hagia Sophia. In my opinion, it’s not worth paying 20TL to enter, but if you have a Museum Pass, go for it.