Mark's paternity?
Did anybody else get a vibe that perhaps Kevin was Mark's father?
If he was, Colette may have decided not to tell him and keep that from him, for her own reasons (like he leads a dangerous life and her life is dangerous enough at home without adding another PIRA warrior to her life as a long term partner or husband).
When Kevin offered Colette something to get Mark a birthday present, it had a certain discomfort to that scene. It's as though Kevin is suspicious Mark may be his son, even though Colette hasn't told him.