During his time in France (1979–1984) he worked in the parish of Notre Dame du Liban (1978–1979) and after that in Saint Medard in Paris.
Khairallah returned to Lebanon, and was appointed Secretary of the Maronite Patriarchal Synod (1985–1987), animator and professor at the Seminary of Ghazir (1985–1989) and pastor of the churches of Kfarhay and Bouksmaya Jebli (1989–1991).
Khairallah was general secretary of the four sessions of the Synod of the Maronite Church (2003–2006) and has held various positions within the Priestly League (1986–2004).
In addition to Arabic, Khairallah speak French, English, German and Italian and also some classic languages, as Syriac, Greek, Latin and Hebrew.
His co-consecrators were Paul-Emile Saadé, the retired Bishop of Batroun, and Joseph Mohsen Béchara, Archeparch of Antelias.