Doom Desire, I've found, is best used as additional support to other pokemon rather than as a straight-up attack for Jirachi itself. Look at these calcs, for example. Timid Max SpA Jirachi using Doom Desire on a few notable threats:
Code:
vs. 252/0 Neutral Metagross: (28.57% - 33.79%)
vs. 252/0 Neutral Suicune: (20.79% - 24.75%)
vs. 252/148 Neutral Cresselia: (30.41% - 36.04%)
vs. 252/0 Neutral Stone Porygon2: (29.14% - 34.49%)
Those seem relatively unimpressive, right? Pair them with the following and see what results you get:
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+1 Modest Sazandora DM vs. 252/0 Neutral Metagross: (54.67% - 64.56%)
LO Jolly Terakion CC vs. 252/252 Bold Suicune: (41.09% - 48.51%)
Jolly ScarfChomp Outrage vs. 252/108 Bold Cresselia: (32.43% - 38.06%)
LO Naive MixMence DM vs. 252/0 Neutral Porygon2: (49.20% - 58.02%)
With Doom Desire factored in, SpecsSazandora can 2HKO Metagross with Draco Meteor (the most specially defensively steel you are likely to see), LO Terakion can 2HKO Crocune with Close Combat, ScarfChomp will 2HKO Cresselia with Outrage (and otherwise does 94% minimum damage total, once SR and 2 rounds of Lefties are factored), and Mixmence 2HKOs Stone Pory2 with Draco Meteor.
The extra 20%-30% that Jirachi provides can really help several pokemon nab key KOs later on in the match, as Jirachi will DD on a counter (say, Heatran), you'll bring in your Sazandora, and then Metagross takes both DM and DD as it switches in. And that's just one scenario. I think the following set could ultimately prove to be one of the most standard support sets for Jirachi:
Jirachi@Leftovers
Bold; 220 HP / 252 SpA / 36 Spe
Wish
U-Turn
Doom Desire
Stealth Rock / Thunder Wave / Iron Head
Loads of team support here. Wish passes a solid 198 HP to any recipient, and a slow U-Turn ensures that the pokemon gets in unharmed while simultaneously preventing Taunt from screwing you up (here's looking at you Erufuun). Doom Desire nabs damage on various threats, as even 2x resists find themselves disliking the added damage. Jirachi can use its bulk to support using the last slot as well, providing either Stealth Rock support, Paralysis, or flinching if you seek that approach. A screen, weather move, or hidden power of choice are all equally viable options, of course.
EDIT: Doom Desire also has the added advantage of taking out death fodder your opponent switches in attempting to sponge an attack from your follow-up pokemon. A minor point, but saving yourself from an extra round of LO recoil or priority every now and then has its advantages.