Hagia Eirini is the first official place of Christian worship in Istanbul (Constantinople), attributed to Constantine the Great and it was used as a patriarchal church before Hagia Sophia was completed. History records mentioned it as the place where the Second Ecumenical Council was held in 381, when the dogma of the consubstantiality of Holy Trinity was proclaimed.
In the Ottoman days Hagia Eirini was incorporated into the palace courtyard but never was it turned into a mosque. In the 19th c. it used to serve as a storehouse for antiquities before these were moved to Cinili Kiosk. At present it is not open to the public so you can only admire its exterior shown in this photo behind the canopy of vegetation of the first Court of Topkapi Palace.