You are my darling

The songlike lines that Bloom spouts about his daughter,

O, Milly Bloom, you are my darling,
You are my lookingglass from night to morning. 
I'd rather have you without a farthing
Than Katey Keogh with her ass and garden,

are in fact based on an Irish ballad, and it seems likely that he used to sing them to Milly as a child. But Joyce received similar lines on a Valentine's Day card, in a prank played by a childhood playmate's father. That context heightens the sense of triangulated desire created by Bloom thinking of the song as he reads Milly's letter about meeting a "young student."

John Hunt 2017

Etching of Samuel Lover in a copy of The Cabinet of Irish Literature (1879) held in the British Library. Source: Wikimedia Commons.

Martello Terrace, a row of eight houses near the beach in Bray, including the green house on the right where the Joyces lived from 1887 to 1892.
Source: homepage.tinet.ie.