It's important to note that "Penfro" is the modern Welsh spelling, while the older spelling was "Penfro". Both refer to the same place.
The name likely derives from the Celtic word *pen* meaning "head" or "summit" and the word *bro* meaning "country" or "region".
Therefore, "Penfro" literally translates to "headland country", which is a fitting name for a county with a long and rugged coastline.