it certainly looks like Dovercastle - does not ring any bells with me
as somewhere around Dublin.
is the entry written by the head of the household or the enumerator ? - I wonder if its a misinterpretation ...
theres a Irish townland placename search at
http://www.thecore.com/seanruad - if you select begins with Dove.. it only returns a few place name, none that sound like Dovercastle
another possibility is that the Dover part is from a place name of Dubh (pronounced dove) meaning black..
e.g. Blackrock = Carrick Dubh
Shane