Afraid to say but the prfix Fitz usually indicates illagitamacy. Ie Fitzherbert's are descended from the bastard son of Herbert or of a Herbert. This is often found in names from Ireland which have their origins after the Henry I landed on the island.
This is true in this way Fitzroy becomes the bastard son of the king. Must be nice to know you have royal blood, even if it is from the wrong side of the sheets! Henry VIII's bastard son was named Fitzroy because of the manner of his birth.