Hi there, this is a pretty standard team, but there's a few issues you need to patch up:
First off, you've got a couple massive weaknesses. Terrakion is a big one, as it can OHKO your entire team. You're prety much saccing a Pokemon every time it comes in, and if it happens to be paired with a CB tar, you're finished, as it can deal with both Gengar and Starmie. For these reasons, I propose you use a
bulky Landorus-T over Gengar. Gengar I see as the weak link, as it doesn't really help you with any threats in particular. Without Substitute or Life Orb, it's going to have trouble getting hits off. It's also really frail. Landorus-T provides you with a much less risky pivot for Fighting-type attacks, can take numerous physical attacks, and take Stealth Rock off Ferrothorn.
Infernape is also a horrendous Pokemon in OU at the moment, as it is simply outclassed and too weak. I think you should replace it with an
NP Thundurus-T, as it features better sweeping capabilities, can hit bulky Water-types hard, which is something your team currently lacks. It can also take advantage of the paralysis support Jirachi provides, which is a huge plus.
With that, you have a team of Landorus-T / Hydreigon / Ferrothorn / Starmie / Thundurus-T / Jirachi. Tbis leaves you with a lack of a revenge killer. To really make your team work the best it can, you need to make one more Pokemon change in using
Choice Scarf Salamence over
Hydreigon. Sorry to change so much, but honestly, Choice Scarf Salamence just fits so well on this team. It can clean just in case Thundurus-T doesn't get to finish its sweep, provides a top-notch revenge killer, and hits many of the same Pokemon that Hydreigon can. Hydreigon is honestly just too slow for this team, as it is forced out too easily. Salamence also gives you a solid revenge killer for Pokemon like +1 DD Nite. It also gives you another way to handle Breloom, which is always nice to have.
With that taken care of, there's a few more changes you should make, in my opinion. First off, you should run a
bulky Starmie over your current offensive one. It allows Starmie to spin much more reliably, but most importantly, it gives you a better way of handling Keldeo. Offensive Starmie can't take any hits from Keldeo, but a more defensive one can, and as a plus it can spin against Keldeo. Secondly, replace
Sunny Day on Jirachi with
Protect, as it would allow Jirachi to have more reliable recovery and thus allow it to deal with numerous threats, especially opposing Thundurus-T, much more easily. Sunny Day is honestly completely useless on Jirachi, especially because it has such 4MSS. Since Landorus-T will have Stealth Rock, replace
Stealth Rock on Ferrothorn with
Gyro Ball, which helps immensely against Latios, Garchomp, and various other opponents.
Good luck!