Mottos in English-speaking countries often draw from Latin renderings (my school, for example, used a Latin phrase which was easily understood by those who didn't speak Latin even though the spelling was different). Isn't it likely that this is the same kind of thing, and the text is based on some older saying which Y'shtola converted to modern Eorzean for the benefit of her travelling companions?

Idyll is already an English word so I don't think the 'Idleshire' thing is anything other than a player joke.