See, the thing about english is that it's really really big. And, as in the case of Samurai, it's borrowing more words from other languages every day.
For all your obscure Gaelic words, chances are you could find distinctive english words, or more recognizable foreign words for which no glossary is needed.
- Dragon age is mentioned above as an example of a setting with few made-up words. And yet consider "The Chantry", used to describe the setting's religion. A real english word, it's evocative, memorable, and unique.
- In an example above, Vartan mentions "'Aulfdar,' a politically loaded title that is comparable to the word 'fuhrer' in German." But why not just use Führer? It's instantly recognizable, and it matches the gruff germanic feel that's characteristic of most fantasy dwarven languages.
- In his Against the Giants series, Gygax called the leader of the frost giants a 'Jarl'. This scandinavian word matches the cultural connotations of the giants, and because it's so close to the english 'Earl', it gives us no pause whatsoever.
As for your specific terms:
Eladrin/Elf[Sblock]Gaelic and Welsh words are neither recognizable, nor particularly easy to spell and pronounce. I'd recommend using them sparingly, and when you do use them, don't be afraid to liberally anglicize--Shee instead of Sidhe, Berrin instead of Bairrfhionn, etc.
Ard Ordu: The Eladrin ruling council
What about using the word 'table' for council? It's unique, but easy to grasp (after all, we have bureaus and cabinets in our government today). I'd personally go with "Table of Lords".
Gol Uirthi: Eladrin earrings that denote caste
I'd just call these "Eladrin Caste Earrings" and be done with it. The concept is unique enough that you don't need a fancy name for it.
Péarla Foirfe: The term for the whole of Eladrin Society
I'm not sure why you need a word for this.
Scáth Amharc Ar: The Eladrin secret police
I'd keep this, or a shortened form of it. It sounds cool, and this sort of organization works fine with a unique name.
Angau Helfa: The elven death hunt
Why not just "The Wild Hunt"? It's a classic.
Rhythid: Elven concept of freedom and destiny
Presumably, elves' conception of freedom and destiny are different enough from humans' that this deserves its own word, in which case Rhythid is just fine. (Though I might tweak the spelling to achieve the desired pronunciation).
Therwyth: Elven druid
I'd probably just stick with "druid" here.[/Sblock]Dwarf[Sblock]German has two advantages when it comes to creating dwarven terms. First, many distinctive german words are recognizable to english speakers. Second, the german and english share a common origin. So look for recognizable german words, and german words that sound similar to their english counterparts (for example, Ghost = Geist, Crown = Krone, etc.)
Atmenschreiben: Ritualistic rune reading
As in divination? I'd just stick with "Dwarven Rune Caster", or somesuch.
Kunstmadchen: Dwarven entertainer-girls
Fraulein is more recognizable than madchen, and judging by the comments, is also more age appropriate. How about "Kunstfraulein", or even "Kulturfraulein"?
Todessuchen: Dwarven rite of passage of wandering the land
What about "Wandernzeit" (wander-time)?[/sblock]Halfling[Sblock]If you're using a language as exotic as Taino for your halflings, I'm guessing that a gap in cultural understanding might very well be the point. I'd use these words, but sparingly, with the expectation that neither the players (and for that matter, their characters) won't actually memorize them.
Cacike: Halfling family leader
Guani: Extended family or clan
Bija: A red dye halfling warriors paint themselves in[/Sblock]