Forgot to add whirfloc / irish moss to wort during boil

If you're not one to care too much about the clarity of your beer, then it won't matter at all . Just proceed as normal on bottling day.

If you do care about the clarity of your beer, your options are to cold crash and use gelatin finings, or just cold crash.

I've hardly ever added clarifying agents (irish moss, whirlfloc, &c.) to my beers. I don't secondary, I regularly do a 2 week primary, then rack into keg. I usually pour perfectly clear beer.

Relax. Don't worry. Have a homebrew.

At this point, neither of those fining agents will work. They both require a good, rolling boil. If you're going for clarity, Your best bet now is to let it mature for another couple of weeks and then cold crash the beer by refrigerating it as near as to freezing as you can get it (without actually freezing it), and let it sit for a couple of weeks/months. You can cold condition the beer in the bottles after they've carbonated, to the same effect as doing it in bulk with the carboy/bucket.

If you are kegging, you can easily add fining agents after chilling the beer such as gelatin or isinglass to help drop out chill haze as well as residual yeast and polyphenols. If dietary concerns are a problem regarding those two agents (kosher or veganism, respectively), you may consider polyclar, just be aware you'll want to rack the beer off the polyclar before kegging. No one likes drinking plastic. Another option of course is to filter, but I assume you, like many home brewers do not own a filtering setup. You may be able to employ any of these fining agents/techniques with bottling, you'll just want to add yeast back into the batch at bottling to ensure there is enough yeast to carbonate the bottles without having to wait a month or two instead of the regular 1-2 weeks.

Here is an article containing all the information you'd need to fine the beer.