I'm very familiar with family lengends, having encountered and investigaed a great many of them. I pay attention to them, and investigate them, because of the possibility there could be a core of truth in them somewhere. Finding it might be helpful for investigative purposes, and would certainly be helpful for purposes of compiling a history of the family.
In almost every case, I've found family legends to be without any basis in fact whatsoever, and in the rare cases where there's been any basis in fact at all, it's been just a small fragment. That many people have cherished these legends, and passed them down through the generations, has no bearing at all on the liklihood of them being true. The great majority aren't!
Thomas Culwell Sr. had a large family. The legend of Thomas Sr. being Irish, appears to be exclusive to descendents and family members of only one of his many children. If the legend was found among several of his children, seperated from one another at an early date, it would give it more credibility. That isn't the case though. The opposite is true. Two other members of his immediate family deny it.
As you've been told, and can verify, two of Thomas Sr's other sons, older brothers of Joshua, and so having known their father longer than he had, testified Thmoas Sr. was born in Virginia. He was their father, and they had reason to know his birthplace. They had no reason to lie about it either! Thomas Sr. was long dead, so whether or not he was ever naturalized was irrelevant.
The legend is unproven and unproveable, and is unevidenced. It's basically a matter of someone having said that Johiua said this. He may have, but the fact that two of his brothers, with it proven they did, said their father was born, not in Ireland, but in Virginia, is what we have to accept. This is he documented testimony of two firsthand witnesses. It can't be ignored, or explained away, because decscendents of Joshua, with nothing to support it, have longbelieved Joshua said otherwise.
It's perfectly acceptable to report family legends, but reqires that it be stated these are unproven stories, and if there's any evidence to the contrary, to make sure people are warned there's sound reason to doubt the story, and it should not be accepted, or reported, as fact.
I'm sorry if I've offended you. I know people cherish family legends, and if they've long believed them, are almost always loathe to give them up. I've been disppointed, and sorry, when I've had to give up a few of my own favorites. I can only say that it was a wise man though who urged us never to forget that history without facts, is fiction.