Aerfort is of course the Irish word for “airport”, but it looks somewhat funny. To start with, that word aer, air, which does not seem to adhere to the “caol le caol, leathan le leathan” principle (and no, it does not – there are other similar examples). And why -fort? Should it not be -phort, because in compound words the second constituent is lenited (p becoming ph), and it is obvious the latter part of this word is “port”? Yes, it should, but for some reason the ligeadóirí agus casadóirí of the Caighdeán have decided otherwise.
One of the problems of Irish orthography is that there is no satisfactory way to write a long e sound both preceded and followed by a broad consonant. In Munster, -ao- is pronounced like this, but in most dialects, -ao- is more like a long i sound preceded and followed by a broad consonant. (A well-known exception is aon with its derivatives, which is usually pronounced with an e sound even in non-Munster dialects.) Note though that this particular long i is very unstable and has very different phonetic realizations due to the influence the broad consonants have on it: the English names Milligan and Mulligan are both based on Ó Maolagáin: in words (mostly names) borrowed from Irish into English the -ao- has very different English reflexes. (Русским изучающим ирландский язык конечно известно – или должно быть! – , что ирландское “ao” – очень похоже на русское “ы”, которое является самым лучшим русским приближением ирландского звука. – Может быть я здесь еще буду публиковать целые статьи по русски, но я стесняюсь писать на этом языке, которым владею гораздо хуже, чем ирландским. Russian speakers have in their native language a very good approximation of Irish “ao” – the sound they write “ы”. I might yet write blog posts about Irish in Russian, but as yet I am ashamed to write Russian, my command of which is much more shaky than that of Irish.)
Long e sounds preceded and followed by broad consonants are found both in loanwords and original Irish words – of the latter, the very word Gael is an example. (Pre-Caighdeán spellings for this word include Gaedheal and Gaodhal.) Note the noun traein ‘(railway) train’. In it. the long e sound is preceded by a broad consonant but followed by a slender one, and to signalize the latter, an -i- is inserted. On the other hand, in its genitive form traenach the -n- is broad, and this is shown both by the fact that the extra -i- is dropped and that the -n- is followed by an -a-, which is unambiguously a broad vowel. Thus, although no textbook I have used makes this explicit, the “ae” of the present orthography must be treated, for all intents and purposes, as a broad vowel in its own right.
Then that -fort. Obviously, the word is a compound of aer and port, and shoult be written aerphort rather than aerfort. However, in the Caighdeán spelling compound words, the last constituent of which is basically -phort, are written like this: aerfort “airport”, longfort “military base, military stronghold, camp”, críochfort “terminal”, calafort “port, harbour” (this is a compound of caladh “port, harbour, landing” and port). This is just a convention, I am not especially fond of it – I would prefer the more regular aerphort, longphort, críochphort – although I admit that caladhphort looks kind of clumsy compared to calafort, and calaphort would feel vaguely wrong.
Note that there is also the word ceannfort, which means “commandant” (there are no majors in the Irish armed forces, there are commandants – I guess this was modelled on the French military tradition, in an attempted departure from the English one). I am not sure about the etymology of ceannfort, but I guess it was originally not a compound word but a genitive construction (ceann an phoirt?). Maybe you’ll find the explanation in Dinneen’s dictionary.