Co Derry is the alternative name for County Londonderry, which is one of the 32 counties in Ireland. So that part of your query is easy enough to answer. There is nowhere in Co Derry called Tonbridge (there is one in Kent, England). The nearest I can think of is Toomebridge on the shores of Lough Neagh, which is on the borders of Antrim & Derry, but most of it is in Co Antrim so that may not be the right place.
You can search all the place names in Co Derry using the following search engine. Enter Co Londonderry from the drop down menu, and then press search, and it’ll print out all the names. (People used a thing called a townland as their address in the 1800s).
McLain is not a common name in Co Londonderry and when you are searching, bear the alternative versions in mind ie McLean, McLane etc, as these are more common. (Nobody bothered much about exact spelling in those days so it varied all the time).