My dictionary says “ní mor do” besides meaning “must” can also mean “might as well”. In some expressions “ní mór do/le” can indicate willingness or deservingness, and “is mór do/le” can indicate unwillingness or begrudging something to someone. Also “ní mór” can be used adverbally to mean “hardly” and “almost”.
I cannot understand any of these verbal sentences - why don't they repeat them - as there's no written help. Especially as the word "ioc" is just being introduced, or did I miss it along the way? also another new word in the previous sentence.. Some written exercises with these new words would be helpful.
ní mór dom is literally "not big to me" - so (very) roughly analogous to "not a big deal", but stronger in meaning. Hopefully that should make it clearer.
Not sure about tá mór dom, but I've never seen it and I can't seem to find any examples. You can say "I must" with the future tense of caith - caithfidh mé..., so maybe you could try ní chaithfidh mé... https://en.wiktionary.org/wiki/caith#Irish
“Is mór do” would be the positive, not tá. But that doesn’t mean the meaning in English would be “must not”. More like stating something IS a big deal or an imposition.