IRS doesn't care. Most likely, neither will someone suing you.
I'm sure you are running your SMLLC with the default "pass-through" tax treatment (not "corporate" tax treatment where it files its own Form 1120). So the IRS considers it a "disregarded entity" and couldn't care less whether the domain is yours personally or in the LLC.
The main feature of an SMLLC is a liability shield. Now, to maintain that liability shield, you do need to conduct the business's affairs as a separate corporate "person", and have "business money and assets" strictly separated from "personal money and assets". Obviously, this should include separate accounting, and the domain name should be carried as an asset on the books, and its cost should have been an expense. However, if you're doing a solid business (mid-5-digits+ annually) and the domain name is one of the only sloppy asset handling instance, that would not threaten your liability shield unless the lawsuit is about the domain name.
Just the same, you should establish a reasonable book value for the domain name (gosh, how 'bout what you paid?) and treat it on the accounting books as an LLC asset, and treat your personal cost for the domain as a cash investment into the business.
You can do that retroactively, since it's basically just a change in your accounting books. It has no effect on your taxes, because it's "pass-through" so the IRS sees it as the same anyway.
Note that the domain name is a business expense; you know what that means for deductibility.