Yeah, I had to study the IPA during my introductory linguistics course but not to the extent of having to be able to read it on the spot. With that said, I had no problem understanding your transcriptions. In Spanish the letter a (like all vowels) has just 1 sound which turns out to be the same you seem to be aiming at, both being "ah" sounds, one longer in Sláine and one shorter in Nechtan.

The "a" in Nechtan doesn't sound like "ah" but like "uh" because it's unstressed...

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne