I think what you need to do, Mary , is go out and buy yourself a good name dictionary. You can find them in the baby and child-care sections of most book stores.
*Name Your Baby* by Lareina Rule would probably be a good one for you, as it lists foreign variations of many familiar names.
The book will show you that the names "Martha " and "Margaret " are two very different names -- sprung from different languages and possessing different meanings. It'll show you the same in regard to the names "Bridget " and "Margaret ", too.
"Marta ", on the other hand, can be a foreign variation of either "Martha " or "Margaret " -- just as "Cindy " can be a nickname for either "Cynthia " or "Cinderella " (two distinctly different names, as well).
If you are serious about this family research that you say you're doing, then do a little additional research of your own into names and name meanings. I strongly recommend that you buy a name dictionary of your own and use it.