Yes, I looked it up and it said it is not necessary to make use of the genitive of measure or the genitive of apposition, except in cases where people are accustomed to seeing a phrase used that way.
For instance, the example you cited 'In Dublin's fair city' is an odd construction that is more commonly phrased, 'In the fair city of Dublin.'
For instance, the example you cited 'In Dublin's fair city' is an odd construction that is more commonly phrased, 'In the fair city of Dublin.' Do you agree?-- No; it doesn't fit the rhyme or the meter.