The mosaic of the cross in the apse of Hagia Eirene in Constantinople is examined in order to determine the imperial patron responsible for it's construction. Key points in this study are Orthodox image veneration, Iconoclast doctrine and the events of the era of Byzantine iconoclasm (726-843 C. E.). Supplementing the written evidence is a study of the architecture of Hagia Eirene and what it reveals about the date of the apse mosaic's construction. Other important monuments discussed in comparison with the mosaic in Hagia Eirene include the apse of Hagia Sophia in Thessaloniki and the now destroyed apse of the Church of the Dormition in Nicaea. The conclusion is upheld, contrary to what is commonly stated, that the Iconoclast emperor Constantine V was not the patron of the apse mosaic in Hagia Eirene; it was the Orthodox empress Irene.