Also, “Tír Chonaill” is an old name for Donegal (Dún na nGall - the fort of the foreigners).
[QUOTE=Jerevan SomervilleSo what we have here are two settlements, one which was built up around the “black pool” and the other around the ford; and in time they grew together and became indistinguishable.[/QUOTE]
There’s a diagramme here showing the relative positions of the two settlements, the ford, the rivers Poddle and Liffey, and the Black Pool.
Does this truly fit the criterion (he asked, fully cognisant of the perils inherent in posing such a question to someone called hibernicus)?
According to my guide to Irish Place Names (referenced above) Dún na nGall was the name of the Danish settlement (now Donegal town), while Tír Chonaill (Tirconnell) was the name of the region, a reference to the “land or district of Connell Gulban, son Niall of the Nine Hostages”.
Which leads me to ask: is either “County Donegal” or “Donegal Town” called Tír Chonaill in Irish today? Or, failing that, was Tír Chonaill ever truly synonymous with either town or county?
If neither is the case, then I’d have to say this example does not fit the OP’s criterion.
I frequently see Co. Donegal referred to as “Tír Chonaill” in modern Irish usage.
Cool. (And I notice you don’t give your location as “Dublin”, ha. 
)
Wendell, at last we have a winner!