The National Trust has a site where you can map names in Great Britain (i.e. England, Scotland and Wales for the geographically challenged). It will do it for two years a century apart, i.e. recently and a century ago, which usually just ends up showing expansion of the name over time. Sorry I don't have the URL, because I lost all my bookmarks.

It's a pity it doesn't cover Ireland (either part). I'm not sure that the spread of Callaghans, O'Callaghans and Callahans in Britain means very much, although it will let you do that. It's a bit like trying to map a Polish name in France, for example. Very useful for mapping Scottish names, though, I would think.