From https://corkirish.wordpress.com/verbal- ... he-dative/

I noticed that the suffixes are all -igh or -aigh.

Is there a general rule on when to use one or the other or do I just have to remember? How would I pronounce these? Would it be wrong to pronounce -aigh as \ee\?

Also, another question: What sound does a "palatized g" make?

-ach -> -aigh
-each -> -igh

preabarnach -> ag preabarnaigh
béiceach -> ag béicigh

Search for "caol le caol agus leathan le leathan", the most important spelling rule in Irish.

Pronunciation depends on dialect.
In Cork probably /ig´/ (with a palatalized g )

A palatalized g (slender g) is a voiced palatal plosive.
Though it is not that strongly palatalized in Irish, it is rather a voiced palatovelar plosive (more similar to "gy" than "dy")

I think g is often slender in English. Slenderisation is not phonemic in English, but it just turns out we have some slender consonants. Eg. in the word "king", both consonants are slender. And in the word "wig" you have slender g. It is in fact hard for native speakers of English to pronounce the broad c and g in Irish, eg. cuin (dogs) is liable to come out as "cin" (= decide, determine).