"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.
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.)
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."
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.