I haven't reached a point in the Welsh course where I've encountered a mutation or shift, which when I first heard about it, I kind of got nervous. Is it similar to Lenitions and Eclipsis from Irish if anyone knows what I am talking about? If so, when does it occur?
Not until lesson 17, it's a bit easier in Welsh as we really only have one main mutation, it's similar to lenitions or consonant softening, anyway it's all explained in the notes of unit 17.
When I started learning Welsh, we tackled mutations early on. I'm glad we did as it gave me confidence when meeting new forms which triggered mutations. Perhaps if you're anxious, you can read up on mutations online and not be intimidated at all when you get to them in the tree :)
I speak Welsh fluently and used this page last year to improve my mutations. (https://en.wikipedia.org/wiki/Literary_Welsh_morphology).