Jill Crewe's pony, Black Boy, was renamed Danny Boy in one of the 1960s paperback reprints (Knight's). I thought that was the one and only time Danny Boy reared his head, but no. Dorling Kindersley published Classic Horse and Pony Stories in 1999, the selection being made by Diana Pullein-Thompson, in which the final chapter of Jill's Gymkhana appears. The use of Danny Boy explains why Neal Puddephatt has drawn Black Boy as bay.
I'm not aware of any other story collections in which Jill makes an appearance. If anyone does, please let me know.