to get there from ubud main road, go west until you find intersection at the end of the main road then turn right. the warung is not far from the intersection, on the right hand side. there are several nasi ayam kadewatan warung but ibu mangku is the original one.
i heard they have branch now in seminyak area, somewhere across bora-bora villa not far from seminyak square. but i’ve never been there.
update: a friend told me they also have another branch in denpasar. in renon area.