No-one's going to mind in the slightest if you lift Gaelic type or script for a D&D setting. (If you start larding in corny or inaccurate Irish stereotypes as well then people might start to be offended and/or amused.) If it matters, the writing style is basically just a long-surviving regional variation of what was once a mainstream form of Latin script, anyway: https://en.wikipedia.org/wiki/Carolingian_minuscule#/media/F... . So its story is quite similar to the story of how blackletter hung on as the primary script family for German until roughly the same time in the 20th century.