Iyo no Jo Munetsugu, first generation, was a student of the first-generation and resided at Isahaya in Province. Tradition holds that his family, bearing the surname Sakai, served for generations as priests at Tenmangu in Nagase Village — the birthplace of — before he moved to forge at Nagase-cho in the castle town of Saga. According to the "Iyo no Jo Munetsugu Genealogy" preserved among the Sakai family descendants, he received the court title of Iyo no Jo in Keicho 11 (1606), a dating corroborated by surviving works from Keicho 12 already bearing that title. Extremely rare dated examples from the early 'ei era constitute valuable reference material. There were several generations working under the name, and those examples bearing reverse file marks are attributed to the second generation.
Among the group of swordsmiths, Munetsugu is a singular presence whose manner of work differs somewhat from the orthodox main line. His and are richly covered in ; he forges vigorous incorporating , , and ; and such activities as and appear freely, with , , and also intermingling. Whereas swords in general temper the straight with a small turnback, in Munetsugu's work the great majority enter in , frequently finishing with strong in a flame-like manner. His construction likewise exhibits distinctive features: a -like form in which the flesh on the side is reduced, and he consistently cuts an idiosyncratic long signature on the in slightly large, fine chiseling. In these respects he deliberately pursued a - domain — especially a -like character — setting him apart from his contemporaries.
Munetsugu is known for excelling in , while appears to be uncommon, though on rare occasions he produced straight-tempered works of high quality. His workmanship is accomplished, and recurring assessments describe an evident spirit of dominance and a fierce, blazing intensity in which one senses the driving spirit of a smith who devoted himself with particular passion to recreating - within the tradition. Representative works display bold Keicho presence — wide in , shallow in curvature, with extended — and exuberant, complex activities extending across and that are brimming with force. He remains an unmistakable and particularly distinctive figure among smiths.