"Off the girl."
Except you wouldn't use 'de(n)' to translate 'the phone of the girl' - you'd use the genitive for that, to indicate possession.
You only need to use 'of' to translate 'de(n)' occasionally, such as 'of' in 'made of' (tá sé deanta de chlocha = "it's made of stone") and in what's called the partitive (indicating some or part of something, such as duine de na buachaillí = "some of the boys").
Generally, take the base meaning of "de" to be "off, from". And remember, the word is not defined by the random collection of English prepositions that happen to translate it.
"And remember, the word is not defined by the random collection of English prepositions that happen to translate it"
Very true for the prepositions in Irish, an excellent point. Most of the prepositions have only a moderate connection to the English preposition commonly used to translate them.
Yeah, adpositions in general tend to shift a lot. They tend to occupy a certain range of conceptual space, and even for closely related languages, the exact boundaries of that space rarely line up perfectly. Even within languages there can be overlap, and across languages, a perfect one to one comparison is virtually never present for even one adposition.
I think it's a pure grammatical exercise, not intended to have a great deal of meaning.
We could invent a context for it, weird though it may be: "Hey, remember that girl we saw who fell asleep during her picnic and there was a spider crawling on her? Is the spider still on that girl?" "I just checked. The spider has crawled away. It is off the girl."
Lenition is a change of an initial consonant sound that is applied in various grammatical situations in Irish. Ablaut in English is akin to it, where a change in a verb’s root vowel sound signifies a grammatical change, e.g. sing/sang/sung. Lenition is expressed in Roman type by following certain initial consonants with an H. (In Gaelic type, lenition is instead expressed by putting a dot on top of those consonants.)
From reading ALL the answers above, including ataltane's answer (the most helpful), here's the answer to your question : BOTH "of" and "off" are correct, even though English language gives two different meanings to these prepositions. You just have to bear in mind that "of", "off" (AND "from") are approximations of the true meaning of "Den" which doesn't have a TRUE EQUIVALENT in English. But that's only my understanding of/off/from the comments I read above... I could be mistaken.
The French "de" comes from Latin "dē”, with the general idea of “away from”, “of”, “about” (prepositions seldomly function alike in different languages). So you can see how that could have evolved into the modern Romance forms.
However, it's not likely that the Celtic languages got the form from Latin. It's more likely that Latin and Irish developed the form from a Indo-European root “de” meaning “towards” and of course the meaning of the connection changed with time. Indo-European is the parent language of most of the European languages, from 4-5000 years ago.