Schematic and basic pattern scanned from OOP book, provided by Isle of Man Museum.

One size only, no needle size or yarn suggestion, only gauge.

Extant example in blue in the Museum - link to museum entry.

Photo from the Shetland Museum, of Manx fishermen.

A good pattern that closely resembles the photographed gansey would be Jorn’s Gansey by Beth Brown Reinsel.

Learn how to sing Manx songs. In Manx.
https://www.culturevannin.im/news_story_636967.html