Question: how to do sirris questline?
Answer: Start: Road of Sacrifices trigger. ... 
Find and give the Dreamchaser's Ashes to the Shrine Handmaid in the Firelink Shrine.
Speak with Sirris at Firelink Shrine and gain the Darkmoon Loyalty Gesture.
Kill the Sulyvahn's Beast at the bridge near the Irithyll of the Boreal Valley bonfire.