Your "Tanzada" or "Tansada" could be a form of the nickname "Tansy" -- a "flower name" that actually comes from the Greek name "Athanasia", meaning "the deathless, or immortal". In 19th century England, the bestowing of classical names on children (especially girl children) was in vogue.
Alternately, the name could come from "Constans", "Constant" or "Constance", meaning "steadfast". I believe there was an early Irish Christian martyr who bore a name somewhat like that (and there's your Irish connection, then), but Daividh would know that better. In any case, the name has been known to be shortened to "Stanca", "Stanza", or "Tanza".
And then, of course, "Tanzada" might even be some completely whimsical invention of one of your ancestors, with no meaning whatsoever to anyone other than to the creative individualist who originally bestowed that name upon his or her child. Eh, people haven't changed that much in the past couple of centuries. :)