McFly Family: Seamus and Maggie are both from Ireland.
Click here This is where I found out the ethnic origins of last names. It turns out that McFly is a real last name. But it's Scottish. But many Scottish names could be considered Irish, like McIntyre or McCoy.
Doc Brown: He said his father emigrated from Germany. His mother's maiden name is Lathrop, which is English.
Clara Clayton: Clayton is an English name.
Lou Carruthers: Carruthers is a Scottish name.
Red Thomas: He could be either English, German, or Welsh.
Tannen Family: I couldn't find out the ethnicity of the name Tannen, but the song "O Tannenbaum" led me to believe it's German.
Baines Family: Baines is an English name.
"Data" Unger: Unger is a German name.
"Spike" O'Malley: I didn't have to look it up to know it's Irish.
"Whitey" Nogura: There were no matches for that name, but I think he's Japanese, judging by his hoverboard, which had a rising sun design. And Nogura sounds like a Japanese name.
Strickland: Strickland is an English name.
Mark Dixon: Dixon is a Scottish name.
Jennifer Parker: Parker is an English name.
Otis Peabody: English