It gets a little complicated. Sorry. Dà as you may have noticed lenites the following noun (puts the h after the first letter) wherever possible (when the noun following doesn't start with a vowel sound, l, n, or r, and what I think of as the gumpot combos sg sm sp and st). Aon does the same, except for when the following noun begins with d, t, or s. I seem to remember this also happens to adjectives following nouns that end in 'n', which is why it is sgian dubh, rather than sgian dhubh in spite of knife (sgian) being feminine. I think it's a euphonics thing.
Yes. It's known as the sgian dubh rule, the dental rule or more generally as the homorganic rule. The principle is that lenition is less likely to occur when the last letter of one word and the first letter of the next are produced in the same part of the mouth. But the details vary from case to case and language to language. The Welsh article (which used to be yr) does not lenite ll or rh, and it is why we have MacGregor rather than
A very interesting, if subtle, point. You may well be correct. Certainly I have always assumed it was the difficulty of moving your mouth rapidly from /n/ to /ð/ (which is how dh used to be pronounced). The problem is that I don't think it is actually that difficult. We have lots of words in English with nth /nθ/ (tenth, month, anthrax) (which is nearly the same and is how n th would have been pronounced) and we don't have a problem. I think we can agree it comes under the heading of phonotactics, but I am not sure we can strictly rule out euphonics, as I don't think anyone really understands the mechanism.
The insertion of the h is a convention to show a set of sound changes called lenition. This is described in the notes and is an important feature of all Celtic languages. In Gaelic specifically it is almost always caused by certain words or classes of words. You may have met it caused by feminine singular words.
The words that cause it are introduced as they arise. The ones that I guess you have met in this unit are aon '1' and dà '2'. So any noun or adjective after these words will take the h if it starts with a suitable consonant.
Duolingo has a deliberate policy of not introducing too many complications all at once so I'm not going to list the letters affected here.
One thing which catches out lots of people is that both of these words affect all nouns regardless of gender.
If you have learnt Welsh note the difference that aon lenites both genders but un only softens feminine nouns. DD