He heard it after he was bitten by Nagini.
He heard it while he was following the silver doe.
He heard it when he was captured by the Snatchers.
He heard it when he was digging Dobby's grave.
Hush now, don't you cry
He heard it when they were escaping Gringotts.
He heard it when they were invading Hogwarts.
He heard it when he was watching Voldemort murder Snape.
He heard it when he was walking to his death.
That's an Irish lullaby
It comforted him, it soothed him, even though he couldn't remember who was singing or even all of the words. It wasn't until later, after the casualties had been reported, until he decided to go back into Snape's memories, wanting to make sure he wouldn't miss anything that would exonerate the man, that he finally found the answer.
In a memory, tucked out of the immediate rush, was one of his mother and Snape at the latter's house in Spinner's End. They were sitting in front of the fire, and this seemed to be the first time the two had seen each other since their fight in 5th year.
"His name is Harry, Harry James Potter," Lily said, holding up her small son, swaddled in blankets, to allow her old friend to see him better. "Do you want to hold him?"
It was on the tip of his tongue to say no, that he wanted nothing to do with the Potter spawn, but this was the first time he had seen Lily in years, and she had finally accepted his apology. He didn't want to ruin that now, especially because he knew this was the last time he would see her until the war ended. So he conceded, holding the child carefully how she showed him.
Sleepy green eyes, so like his mother's, regarded him from under a fringe of messy black hair. He yawned, fussing a little, and before Lily could do anything he gently began to rock the child, remembering a lullaby his mother had sang when he was a child, something to take his mind off of his father's drunken ravings. So, looking into the child's green eyes, Severus began to sing softly in his deep baritone, hoping to quiet Harry so Lily wouldn't leave so soon.
Too-ra-loo-ra-loo-ral, Hush now, don't you cry,
Too-ra-loo-ra-loo-ral, That's an Irish lullaby.
He could only remember the chorus, the rest of the words forgotten long ago, but it was enough to quiet little Harry, who curled a little closer to Severus's warm chest and drifted asleep before Severus had finished repeating the chorus.
Lily was pleased, and she stayed a little longer, which pleased Severus. He never spoke of the moment again, nor of the fact that when he went to Godric's Hollow, after his tears had abated somewhat and before Hagrid came, he sang the lullaby to Harry, calming the child enough that when Severus left and Hagrid appeared, he wasn't scared.