Every beginner family researcher learns quickly that some names and some families are easier to research than others. In my own family tree, the Jones family of Carmarthen are a nightmare: over 500 Mary Joneses in Carmarthenshire for the early 19th century, and nothing much to distinguish one from another. However, I also have Earwakers and Jearrads in my family tree, both quite unusual names, and much easier to research.
In most cases, you can’t be sure where in the country a name first arose. Names relating to physical properties of the land, like Hill, Lake, Undercroft, definitely arose in different places. Names relating to professions, like Clark, Smith, Butcher are the same. But when you have a very unusual name it can be the case that it is associated very clearly with a particular location.
At this point, I should sound a warning, because spelling of names wasn’t always as fixed as it has become over the past couple of centuries. Shakespeare, for example, used seven different spellings of his name, and used them interchangeably. Before the dictionary was invented, it didn’t seem important to people to spell the same words the same ways all the time, as long as they were understandable, and the same seemed to go for surnames. So your Jearrad could be Gerrard or Gearad or Giarrad, or a misspelling of Gerald, and not related to a family who spelled their name the same way, and the only way to know for sure is to trace each generation back.
In my own family, Clarke had been spelled with an ‘e’ for every certificate, every event in the family, back to the death certificates of my great-grandparents, and when I first started researching my family 29 years ago, I rejected people with no ‘e’. But I discovered some time later that, for some reason, my great grandparents’ marriage was registered as Clark, whether by mistake of the registrar or lack of clarity from my great grandpapa, and so I only discovered his marriage by being open to the idea it might be spelled differently. The further back you go, the more unusual the surname, the better this advice becomes!
There are one-name studies for a lot of surnames, and the guild of one-name studies has a list of them. You do not need to join the Guild to see the list of surnames, but you will need to join the Guild in order to see information at a deeper level in the indexes for the various names. Membership is currently (January 2018) £18 for membership until 1 November, 2018 and £32 until 1 November, 2019, but it reduces on a sliding scale as the months pass. Each study has a contact who well may be able to give you information about the family you are researching.