as far as I can tell cha and chan eil are both used to indicate a negative statement but they get used in different contexts.
Cha on its own I'm not sure about, but "cha toil leam" is used when the positive statement would be "is toil leam" (I like… or, I don't like…)
"Chan eil" gets used in a more general "this/he/she/that is not…" sense, where the positive version would be "Tha".
Thus: Is toil leam buntata. Cha toil leam buntata. (I like/dislike potato).
Tha seo dona Chan eil seo dona (This is/isn't bad)