"I sleep on the airplane."
Translation:Codlaím ar an eitleán.
Yes, this is because of the preposition.
The technical explanation is that prepositions cause the following noun to switch from the familiar version (called the nominative case) to something called the dative case. These two cases are almost always identical, but one difference is that words beginning with vowels have no initial mutation.
- after a (“her”);
- after a dhá (“her two”);
- after cá;
- after Dé (“day”);
- after go (“to”) or le (“with”);
- after na (both nominative plural and genitive singular feminine);
- after Ó;
- after ordinals except chéad;
- after trí, ceithre, or sé with uaire.
- after a (in counting);
- after chomh, go, le;
- after na (both nominative plural and genitive singular feminine).
- after cé, ní, le.
- after ná.
"tá mé I mo chodladh" can also be translated as "I am asleep" - it might even be a better translation than "I am sleeping", as the gerund is usually translated with "ag" and the verbal noun.
But your point still stands - "I am asleep on the plane" isn't the same as "I sleep on the plane".
To get the habitual sense of "I sleep", while still using the "i mo chodladh" structure, you could say "Bím i mo chodladh ar an eitleán".