Snap judgement was that it was Gemma, but I'm starting to doubt that.
Biggest questions I have going into next season:
Spoiler: (Highlight this box to see the hidden message.)
1. How is club going to handle protection while Tara is inside
2. What role Donal Logue's ex-marshall charater will play. (They wouldn't have introduced a character and established a background for just two episodes.)