Rabbi Isaac Tyrnau (יצחק אייזיק מטירנא or יצחק טירנאו; also Isaak Tyrnau) was an Austrian (or Hungarian) rabbi, born in the late 14th century and active in the 15th century; he is most famous for his Sefer haMinhagim (Book of Customs).
Gabriel Polak and Israel Böhmer published (Königsberg, 1857) an anonymous story entitled "Etzba Elohim," the heroes of which are Isaac Tyrnau and his beautiful daughter.
The number of scholars had diminished so much that in some places "there were only two or three persons who had a real knowledge of local customs".
Tyrnau's work is noted as the first to discuss in detail the idea of the Yahrzeit (the commemoration of the anniversary of a death).
It also contains a treatise on morals entitled "Orhot Hayyim," in 132 sections,[1] which is appended to the Sefer haMinhagim.