An IRSN is an Internal Revenue Service Number that is assigned to a taxpayer in case the tax return was filed with no SSN/ITIN.
Please keep in mind that an IRSN is not a valid taxpayer ID and it cannot be used for filing a tax return. The only purpose of it is to temporarily register your tax return into the IRS system for processing purposes. You need a valid taxpayer ID (SSN/ITIN) in order for your tax return to get successfully processed.