You have to identify a specific cause of action, what will it be? There is no statutory requirement that the IRS process your tax return and pay your refund in any particular time frame. Therefore, there is no law you can sue them for breaking, and no deadline you can sue them for failing to honor. Even under a theory that the government is required to conduct all its business in a timely manner, you're going to have a difficult time arguing that timeliness is breached given the unusual circumstances.
The IRS is required to pay interest on refunds that are paid more than 45 days late, and they are so paying.
