Ryansdóttir is a surname of Icelandic origin, meaning daughter of Ryan. In Icelandic names, the name is not strictly a surname, but a patronymic

Patronymic

A patronym, or patronymic, is a component of a personal name based on the name of one's father, grandfather or an even earlier male ancestor. A component of a name based on the name of one's mother or a female ancestor is a matronymic. Each is a means of conveying lineage.In many areas patronyms...

 (see Icelandic names). Since Ryan is a rare name in Iceland, the number of girls with the last name Ryansdóttir is extremely rare. Seen slightly more frequently is the patrynomic, Ryansson, which would mean the son of Ryan.