Where does “Gaelism” come from?

Gaelism (English) comes from English Gael, from Irish Gael, from Irish Gaoidheal, from Middle Irish Gaídel, from Old Irish Goídel, from Old Welsh Guoidel, from Proto-Brythonic guɨðel, from Proto-Celtic wēdus — forest, wood; tree, beam.

Ancestry of “Gaelism”, step by step

Gaelism traces back through two separate lines of ancestry.

via English Gael

StepLanguageWordMeaning
1EnglishGaelA member of an ethnic group in Ireland, Scotland...
2IrishGaelGael, Irish person; Highlander
3IrishGaoidheal
4Middle IrishGaídel
5Old IrishGoídelGael
6Old WelshGuoidelwild man, warrior
7Proto-Brythonicguɨðelsavage, woodsman
8Proto-Celticwēduswild
9Proto-Indo-Europeanwidʰu-forest, wood; tree, beam

via English ism

StepLanguageWordMeaning
1EnglishismAn ideology, system of thought, or practice that...
2English-ismUsed to form nouns of action or process or result...
3Latin-ismus-ism
4Ancient Greek-ισμόςForms abstract nouns
5Ancient Greek-μόςForms abstract nouns
6Proto-Indo-European-mósCreates action/result nouns from verb stems
Every word from Proto-Indo-European widʰu-Every word from Proto-Celtic wēdusEvery word from Proto-Brythonic guɨðel