He was born at Bologna about 1050.
At the instance of Countess Matilda of Tuscany he began to devote himself to the study of jurisprudence, taking the Justinian code as a guide. After teaching jurisprudence for a short while in Rome he returned to Bologna, where he founded a new school of jurisprudence in 1084 or 1088, which would rival the law school of Ravenna.
Some jurisprudence had been taught at Bologna, before Irnerius founded his school, by Pepo and a few others. He introduced the custom of explaining the Roman law by means of glosses, which originally were meagre interlinear elucidations of the text. But since the glosses were often too extensive to be inserted between the lines of the text, he began to write them on the margin of the page, thus being the first to introduce the marginal glosses which afterwards came into general use.
After the death of Pope Paschal II, he defended the rights of Emperor Henry V in the papal election and upheld the legality of the election of the imperial antipope Gregory VIII. After 1116 he appears to have held some office under the emperor
He died, perhaps during the reign of the emperor Lothair II, but certainly before 1140.
Irnerius taught along lines firmly established in the teaching of Scripture, by reading aloud a section of the civil law, which the students would copy, and add to the text his commentary and explanatory glosses. Thus he was the first of the glossators, whose explications of the law became an essential part of the legal curriculum.
The text of Justinian's Pandects used in Bologna, referred to as the Littera Bononiensis, closely parallel to the Littera Florentina, would be disseminated throughout Europe as students returned home from Bologna: there are versions of the Bolognese Littera with provenances in Paris, Padua, Leipzig and at the Vatican (Purpura 2001).
According to ancient opinion (which, however, has been much controverted), Irnerius was the author of the epitome of the Novellae of Justinian, called the Authentica, arranged according to the titles of the Code. His Formularium tabellionum (a directory for notaries) and Quaestiones (a book of judicial decisions) are no longer extant (EB).
His chief work is "Summa Codicis", which is of a special historical value, because it is the first medieval system of Roman jurisprudence. It was edited with a critical introduction by Fitting, "Summa Codicis des Irnerius, mit einer Einleitung" (Berlin, 1894).
Another important work generally ascribed to Irnerius is "Quaestiones de juris subtilitatibus". It was also edited by Fitting, "Quaestiones de juris subtilitatibus des Irnerius, mit einer Einleitung" (Festschrift zum 200jährigern Jubiläum der Universitat Halle-Wittenberg, 1894).
Other juridical works and glosses that are ascribed to Irnerius are extant only in fragments, or their authorship is uncertain.
Irnerius was largely forgotten; his name was revived by German historians of the later 19th century. His name is also seen in manuscripts as Hirnerius, Hyrnerius, Iernerius, Gernerius, Garnerius, Guarnerius, Warnerius, Wernerius, Yrnerius. He called himself Wernerius when he signed documents.
Anders Winroth questioned much of the received account for Irnerius' life and questioned his importance for the history of Roman law in the Middle Ages.