Who did tell Doran about Cersei's plan to kill Trystane?

[Odej]

Cersei planned to kill a lot of people in AFFC including Trystane Martell which was known to Doran. He tells the sand snakes that Dorne still had friends at court who tell him things he shouldn't know. As it seems that only the people directly involved in Trystane's assassination attempt knew about it, Doran's source is very good. Who would it be?

The Merryweathers? Varys? Somebody else?

[Floki of the Ironborn]

The Merryweathers are probably Blackfyre sympathizers with the Golden Company, so unless they're playing more than one game at the same time, I doubt it's them.

Varys would want Cersei to destabilize the realm as much as she could, and telling Doran about the plot to kill his son would definitely have that desired effect. I'd guess it was him.