An Irish company carries more than one number, and they come from different places. They get confused constantly, though the difference is simple once you know who issues what.
CRO number
The company registration number, assigned by the Companies Registration Office when the company is incorporated. Digits only, no letters, and no fixed length: numbers are issued in sequence and have been since the 1860s, so a company incorporated then carries two or three digits and one incorporated this year carries six. There is no eight-digit rule and no zero padding — a CRO number is what the register issued, and padding 12965 to 00012965 asks the register for a company that does not exist.
It never changes — not on a change of name, of registered office or of ownership — and it is not reissued after the company is dissolved. That is what makes it the reliable way to identify a company years later, and it is why every company URL on this site ends in one.
The numbers Revenue issues
Revenue, not the CRO, handles tax. A company registered for tax has a tax reference number, and a company registered for VAT has a VAT number on top of that. They are separate registrations: being registered for corporation tax does not make a company VAT-registered, and a company below the VAT thresholds may have no VAT number at all.
An Irish VAT number is seven digits followed by a letter, sometimes with a second letter after it. Older numbers may carry a letter, + or * in the second position instead — both shapes are still in circulation. For a counterparty in another member state it is written with an IE prefix.
So
Every company has a CRO number. Not every company has a VAT number. And no number here can be derived from any other — the numeric part of a VAT number is not the company's CRO number, and treating it as one will match you to the wrong company.
How to check them
Check a CRO number in the Companies Registration Office's own company search: enter it and see whether the company that comes back is the one you expected. Check a VAT number in VIES, the European Commission's service, which answers for the whole EU and tells you whether the number is valid right now. Both are free and take seconds.
One caution on VIES: it answers three ways, not two. Valid, invalid, and did not answer. A member state's service being down is not a finding about your supplier, and any tool that reports it as “invalid” is telling you something it does not know.