What is "fianshruth"

The title Fianṡruth (Find) refers to two alphabetically arranged Middle Irish lists of names associated with the Finn Cycle, preserved only in the Yellow Book of Lecan and probably datable to the twelfth century. The lists A and B are preceded by almost identical introductions. Many of the approximately 170 names do not occur elsewhere in the attested sources, while a number of familiar faces from later tales in the Finn Cycle, such as Fergus finnbél, are missing.