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The boundary seems to be incorrect here: Way History: Carheeny River (1453944573) | OpenStreetMap I think swapping the geometry of the two ways (for that 50 metre section only) might fix it. Note that one way is connected to 60 relations and the other has none.
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Separately, is this boundary set up differently to the boundaries that Connacht and Munster have with Leinster? The below images are of Overpass-Turbo, searching for admin_level=5 with “Don’t display small features as POIs.” switched on or off. It might be down to whether both the relations and ways are tagged with admin_level=5. Tags | OpenStreetMap Taginfo Ireland and Northern Ireland
It appears that the Carheeny River splits (unusually) around an island at that point, is that correct? (How it hasn’t silted-up one leg and created an oxbow lake, I can’t figure out…) And according to what I can define from the British War Office maps, the border between the two follows the lower/Southerly section of water, is that how you see it?