Complex sentence: Marty also meets his great-great paternal grandparents, Seamus and Maggie when he was stranded in 1885, and also their infant son William, Marty's great grandfather.
Split into simpler sentences: Marty also meets his great-great paternal grandparents Seamus and Maggie, when he was stranded in 1885. He also meets their infant son William, Marty's great grandfather.