You may have to kill Deacons of the Deep to receive the "small doll" to pass through the barrier at the end of the starting bridge in Irithyll of the Boreal Valley, then reload the area.Make sure that she left Firelink Shrine after you've given her the Dreamchaser's Ashes and received the gesture, if you travel to Irithyll right after talking to her the sign will not be there yet.If her sign continues to not appear, completely resetting the game has been shown to solve the problem on occasion.You don't actually have to kill the Sulyvahn's Beast, just cross the bridge and her sign should appear once you reload the area.If you haven't crossed the bridge and lit the bonfire on the other side, her sign may not appear. If you've already killed the Cursed-rotten Greatwood and her sign doesn't appear, warp back to Firelink after activating the bonfire after the bridge then go back to the beginning of the bridge.If her summon sign does not appear killing the Curse-rotted Greatwood makes it appear.
You only need to light the second bonfire across the bridge, teleport back to first bonfire from there and you'll see the sign. For the Creighton the Wanderer interaction you don't need to kill Curse-rotted Greatwood.(*note* You have to light the Central Irithyll bonfire to get the sign to appear.) Sirris's Sign Location
Use it to be summoned into her world and help her kill Creighton the Wanderer. Reload the world and find her summon sign on the bridge around where the Sulyvahn's Beast spawned.