AncientPower is a decent move on Jirachi, has a 20% chance to raise all stats, and below-decent Base Power.
However, it is a bit unusual to run a AP Jirachi, as that is not the best way to abuse Serene Grace.
Most Serene Grace Jirachi's carry: Body Slam + Iron head (known as the ParaFlinch Jirachi).
AncientPower will be unexpected from you, but jirachi gets Calm Mind,(assuming you are gonna use him a Special Attacker) meaning that you can just boost your SAtk and SDef and not run the risk that 80% of the time, you will not get a boost.
Personally, I just love ParaFlich Jirachi, but if you want to be original, you can try to make a set that works with AncientPower and go for it.
