Is this supposed to mean they fight against violence or using violence?
The teanglann entry for foréigean includes a sub-entry for the phrase "le foréigean":
Le ~, by force; violently.
So it seems that "using violence" would be the meaning.
Now there is an ambiguous statement
Why is foréigean not lenited after le?
le doesn't lenite, it prefixes an 'h' before a vowel (in some dialects)
Well then, that would make a lot of sense. Maybe I'm thinking of lena mhadra, but that is only where a means "his"...
Why doesn't foréigean follow the caol le caol spelling rule?
It is a compound word: for- éigean. Compounds don't generally modify the spelling to comply with leathan le leathan, caol le caol.
Thank you. I wondered if it might be