Mark was a brief boyfriend of Debbie Gallagher's during Series 6. Debbie met Mark at a club one night when she was out with Maxine Donnelly and after a night of flirting and heavy drinking Mark took Debbie back to his student residence. Debbie woke up the next day to find herself in his bed, with no memory of what happened after meeting him. Assuming they had sex and with no Mark in sight, she slipped out of the room and assumed she would never see him again.
However sometime later Debbie met him again at the funeral of a family friend that turned out to be Mark's grandmother where he assured her they hadn't had sex that night and he had slept on the floor. Debbie had already made every effort to convince Frank the funeral was at another church to avoid him embarrassing the family but he managed to find the funeral and do just that. Frank got increasingly drunk and told a man holding a hooked up microphone how he had sex with Mark's grandmother at the age of 15 before a bad stomach and occupied bathroom led him to relieve himself behind a tree that was in full view of everyone there.
Debbie conveniently refrained from pointing out that Frank was her father until she met Mark at The Jockey. Frank accidentally punched Mark in the face when he tried to get Frank to apologise for his actions at the funeral, Frank then stormed off but while Debbie was kissing and cleaning up Mark's face he made a throw away remark about getting Frank back. Debbie admitted that he couldn't because Frank was her father but this only angered Mark, as he felt she had been lying to him. Mark's reaction was enough to anger Debbie herself, who despite knowing what her father was like was always fiercely loyal to him.
Debbie admitted having strong feeling for Mark to Maxine, telling her she could even love him but when he arrived at the house later to apologise she slammed the door in his face. Apparently prepared for this, Mark took a chair he had brought with him and sat in the road in front of the house for hours waiting for Debbie. Debbie continued to ignore him until Mark used the chair to knock out a rogue hitman who was about to shoot both Mimi and Paddy Maguire. This heroic act was enough to spur Debbie into taking Mark inside the house and having sex with him on the kitchen floor in exactly the same spot as her older sister Fiona had sex with her later husband Steve McBride.
It can be assumed from Debbie's mention of loving Mark and the fact they had sex in the same way one of the show's greatest love stories started that Debbie and Mark's relationship was set to progress into something big. However, in Series 7 Debbie left Chatsworth to join the army and though she kept in occasional contact with her family, never returned.