Jirachi is def viable, and I'm not just saying that to try and get psychic-ceus from you.
Minority Suspect
Arceus-Pathetic is mine, hands off (gives you evil stare) <3.
I know that Jirachi can do things, but the set I worked up was careful full investment, and it was such Ho-Oh/Yveltal bait that I basically ended up with way too many options. Jirachi obviously wants Iron Head, then it wants recovery in Wish, then it wants Protect, then it wants a paralyzing move (body slam/thunder wave), then it wants Toxic as well, then it wants U-Turn, then it would like an elemental punch or two so as to not give Zekrom/Scizor free switchins, then it would like to be able to use Stealth Rock itself. this is besides the fact that it fails to 2HKO bulky Geoxern with Iron Head, and Substitute can lead to Jirachi getting screwed if it tries to Thunder Wave, not to mention the fact that Thunder/HP Fire/Focus Blast at +2 2HKOes it anyway. it also can't take Spout despite its investment, Yveltal and Ho-Oh make it sad (although neither appreciate status on the switch). It also loses to Rest Palkia one on one as it can't flinch with Iron Head unless it's EXTREMELY lucky. It's setup bait for several SD Arceus formes. thanks to rain no longer being permanent it can't wall certain Dragons (Palkia can Hydro Pump it in rain and Fire Blast it in sun). Did i mention that Mega Gengar traps it and gleefully Shadow Balls it to death, whilst Jirachi can't paralyze Gengar (taunt stops Thunder Wave, Gengar is immune to Body Slam). also, the Perish Trapper variant of Gengar makes Jirachi sad with Disable, Taunt etc.
The other issue is choosing between Thunder Wave and Body Slam. Thunder Wave is probably the better choice as you shouldn't be staying in to paralyze Ground Types anyway, but then you get Taunted and set up on/slaughtered. and if you do run Body Slam then GeoXern can set up Substitutes on you and play games with you too. also, if you don't run Toxic (and finding room for Toxic can be HARD), CM Arceus-Electric sets up in your face and drives a train over the rest of your team.
I'm not saying that Jirachi is unviable, i just feel that outside of checking Xerneas it doesn't really accomplish much and is easy prey for a large amount of the rest of the tier. I'm sure that someone with more experience than I with Jirachi (i only used it in about 20-25 matches today) can do a much better job than I can and give the Wishmaker the analysis it deserves.
P.S. don't use Scarf Jirachi/Band Jirachi/CM Jirachi/any kind of offensive Jirachi. They all really REALLY suck.
P.P.S. don't forget the loss of Dark and Ghost neutralities, it's huge for Jirachi this generation and pretty much nails it right where it hurts (why do you think Jirachi isn't even in OU by usage). I love the Steel/Psychic fairy but sadly it's a lot worse this generation :(