The painting that Tiffany 'enters' in this book really exists. It's called
What people 'know' about Richard Dadd is that 'he went mad, killed his father, was locked up in a lunatic asylum for the rest of his life and painted a weird picture'. Crudely, that's all true, but it's a dreadful summary of the life of a skilled and talented artist who developed a serious mental illness.
A Nac Mac Feegle does not appear anywhere in the painting, but I suppose it's always possible that one was removed for making an obscene gesture. It's the sort of thing they'd do.
Oh, and the tradition of burying a shepherd with a piece of raw wool in the coffin was true, too. Even gods understand that a shepherd can't neglect the sheep. A god who didn't understand would not be worth believing in.
There is no such word as 'noonlight', but it would be nice if there was.