Ogham, Ireland's earliest known alphabet, originated around the 4th century CE. While it began as a script for monumental inscriptions, by the medieval period, it had evolved into a tool for encoding messages, serving as a cipher in various contexts.

Scholarly Use and Cryptographic Applications

In the medieval era, Ogham was employed not only for writing but also as a cryptographic tool. Scholarly works, such as the Auraicept na nÉces, preserved in the Book of Ballymote, delve into the structure and usage of Ogham, indicating its role in encoding and decoding information.

Additionally, the Irish legal manuscript tradition showcases the use of Ogham alongside other cryptographic methods, reflecting a broader concern with abstruse writing among the late medieval learned classes. 

Manuscript Inscriptions and Marginalia

Ogham inscriptions appear in various medieval manuscripts, often in the margins or as annotations. For instance, the Codex Sangallensis 904, dating from the 9th century, contains eight Ogham marginalia, providing evidence of its use among Irish scholars during that period. 

Similarly, the Berner Ogham-Zeichenübersicht, found in Codex 207, Corpus Grammaticorum Latinorum, folio 1b recto, is a seven-line Ogham inscription dating to the late 8th or early 9th century, further illustrating the script's continued use and its cryptographic applications.