Father Callahan was a priest and drug dealer who worked for Tommy Egan.
He appeared as a supporting character in season 3 and the supporting protagonist in season 4.
Tommy meets with Father Callahan and convinces him to join his drug network. When Tommy starts working for Milan he is ordered to kill Callahan because he is selling guns to their rivals the Albanians. Tommy refuses and instead threatens Callahan to stop selling to the Albanians or he will die. Callahan agrees after being threatened. A while later Callahan and the other gang leaders in Tommy's network attend a meeting at Ghost's nightclub to discuss the new distribution plan made by Tommy's new boss Milan. All the gang leaders are upset by it. They all begrudgingly agree to it after Jae Shin is stabbed to death for refusing to go with the plan.
After Ghost is wrongfully arrested for the murder of Greg Knox, Callahan and the other gang leaders in Tommy's network meet up to discuss what they are going to do. Tommy tells them to relax and that everything will be fine. Callahan continues to work for Tommy until Dre tells everyone in Tommy's network that Tommy wants to go to war with the Jimenez Cartel. Callahan and the other gang leaders in Tommy's network leave his organization after hearing this. Tommy later meets up with Callahan and tells him Dre ordered Julio to die. Callahan meets with Dre and tells him he wants to leave his organization. Callahan tells Dre he knows he set up Julio to die and wants nothing to do with Dre. Dre stabs Callahan in the chest after Callahan tells him this.