Kate Greenaway was one of the first illustrators of books for children. She illustrated nursery rhymes and verse and her books were sort after. They are still sort after, but now by collectors, not children. For the children of today, she is probably known more for the prestigious award that has her name attached to it. The Kate Greenaway Medal was established in 1955, for distinguished illustration in a book for children.
While the award is administered and awarded in England, two Australian illustrators have won it. In 1995 Gregory Rogers won it for his illustrations of Libby Hathorn's Way Home. This book about a homeless boy was quite controversial when it was published because of its theme, but it was never intended for a very young audience and it is a book that elicited much discussion among the audience it was intended for. I had the honour of working on the teachers notes with a colleague and got to know the book well. The other winner is Bob Graham who needs no introduction and who is a well-deserved recipient. He won for Jethro Byrde, Fairy Child.
If you are looking for books to celebrate St Patrick's Day that are not factual, but literary, I think I would look for something Irish like Jude Daly's Fair, Brown and Trembling: An Irish Cinderella Story or something by Tomie de Paola like the Irish folk tale Jamie O'Rourke and the Big Potato. Apparently, de Paola even has a picture book about St Patrick, but I haven't seen it. Perhaps your library will have it.