The boundary for the barony's "Clanmorris" and "Kilmaine" do not follow along this short segment of river but follow the road (located east of marker) south and loosely follow the smaller road (south of marker) to the west. You can see untagged line segments (in the editor) which belong to the correct boundary. It then joins the boundary seen to the south west of this marker.
This is a major boundary, so it gets a bit confusing. There also seems to be a civil parish "mayo" overlapping the two baronies which is strange. I think this resulted in the strangely named civil parish "Mayo (clanmorris portion)". I think it should be just "Mayo".
. Source: "Ireland Civil parishes and barronies - memorial atlas"
Any thoughts??

Missing civil parish boundary along here. It should be the parish of "Balla" to the north and "Mayo" to the south. Instead they are merged as one parish called "Mayo (clanmorris portion)" . What is the source for parish boundaries in OSM anyway? I don't know where 'clanmorris portion' came from.