Hi, cain 22. I'll be doing an article on Jirachi eventually, but to briefly answer your question: it's inspired by the Japanese tanabata festival. Tanabata is a star festival (hence Jirachi's shape) and an important part of it is writing wishes on small pieces of paper, and hanging them from bamboo. These wishes are the little tags that hang from Jirachi's head.
As for its Steel typing... I haven't yet found a good reason for this. I'll keep looking, but for now it seems like it was made a Steel type just because (much like Mawile).