Just realized...I didn't offer an answer to your basic question!
As I indicated, I've begun studying again after many years of not studying. My original textbook was called "Teach Yourself Irish." A girl I knew in Cork saw me reading it, and she said, "You'll never learn Irish readin' that!" She brought me her Buntús Cainte books from school, and also gave me a couple of other books to help. I bought some dictionaries on my own, and a novel.
One of the online resources I really like is Irish Dictionary Online, which is at www.irishdictionary.ie. One of the things I like about it is that it gives many examples of sentences with the words it brings up, in most cases. For difficult constructs, it is a great blessing because sometimes a word just doesn't exist in Irish, and you have to learn the idiom for the same expression in English.
I looked up the word fada in the online dictionary, and it gives the definition as "long" of course, but also includes "far, lengthy, protracted", and all as adjectives. While the word fada is used to refer to the diacritical mark giving the long sound to a vowel, it is not, apparently, considered a noun and does not have a nominative form. The word fad is the noun for length, distance, or duration.
In the sample sentences under fada in the dictionary was this offering: tá cosa fada uirthi, She has long legs. While the word cos "leg" is pluraled to cosa, fada remains the same, so I assume it is singular or plural, or doesn't have a discrete plural form. Of course, when writing in English, as you were, adding the "s" to give the plural would seem correct, but would also convert it into a noun.
I suspect seano (John Dorins), another member of this forum, can give us the low-down on this item. I hope he will see this. I suspect that the use of the term "fada" to refer to the long mark will be properly expressed in Irish as comhartha fada "long mark", but I'm not certain of that. I know that, conversationally, it is used as a noun in English, at least, so I don't know that it hasn't developed a nominal sense in Irish at this point, either.
So there you have it, until someone comes along who knows what he's actually talking about.
I live in California in the USA, but spent two years in Ireland in my early twenties. I envy you going to the Gaeltacht. I'm afraid I'll never quite get there. I hope we will get to go to Ireland some time, but life has a way of putting things off. I did give Irish first names to all three of our children, but I gave them English middle names, just in case they decided they couldn't stand the Irish ones. And we named our dog Oisín. The woman we adopted him from had called him "Francois" but we realized right off that it wouldn't do for a dog in our family. He seems to like the new name better.
All the best to you; I hope you find some people to skype with. I've skyped a couple of times, but I'm such a beginner at speaking the language. I'll keep working at it, though.
Dale D