When you kill "Ghastly Siabras" your faction doesn't go up or down. I've been killing siabras all day and I have yet to see "Your standing with the Siabras has decreased."
Ok, if you follow the shoreline from town... as you get near the forest.. the path banks right. All alog from the hill line to your left and all the way down to just before the maiden area in the woods is where you will find ghastly AND "Ghostly" Siabras. (yes there are other spots, but this is a main concentration). from the shoreline, you should not have to go deeper than the moss men area. At night the ghastly and ghostly siabras will spawn. During the day you will see red wolves and dergas. Also to point out, in the afore-mentioned area, at night will sometimes spawn, "Fallen-Ones". *15+ mob*
There are only ghastly's that I see there, and they con orange at level 14. There are no ghostly siabras there. Also, the siabras are surrounded by dergans, so getting access to a siabra will require patience.
I think there are some in East Lough Derg. From Mag Mell, go towards the water (east, right?), follow the shoreline a ways until it takes a sharp right. In that forested area that is directly in front of you, I believe there are both ghostly and ghastly siabras.