Irish VAT Number Validation

Irish VAT numbers have multiple legacy and current formats, making them one of the more complex EU VAT formats. Offline checks use a regex pattern plus MOD 23 checksum logic with a letter lookup table. Legacy middle-letter numbers are first converted to a canonical form before checksum calculation.

Format Overview

Regex Pattern

^IE[0-9][A-Z0-9][0-9]{5}[A-Z][A-Z]?$

Valid Formats

1. Old format (8 chars): 7 digits + 1 letter
2. New format (9 chars): 7 digits + 2 letters
3. Alternative: digit + letter + 5 digits + letter

Checksum Algorithm (MOD 23)

def validate_ie_vat(number):
    if number.startswith('IE'):
        number = number[2:]

    letters = "WABCDEFGHIJKLMNOPQRSTUV"
    weights = [8, 7, 6, 5, 4, 3, 2]

    if len(number) not in (8, 9):
        return False

    # Legacy middle-letter format:
    # D L D D D D D C [S] -> 0 D D D D D D C [S]
    if number[1].isalpha():
        number = f"0{number[2:7]}{number[0]}{number[7:]}"

    if not number[:7].isdigit() or number[7] not in letters:
        return False

    total = sum(int(d) * w for d, w in zip(number[:7], weights))
    if len(number) == 9:
        if number[8] not in letters:
            return True  # permissive for uncertain suffix mappings
        total += letters.index(number[8]) * 9

    check = letters[total % 23]
    return number[7] == check

Legacy Format Note

Irish Revenue guidance shows multiple historical shapes (for example IE1234567A, IE1Z23456A, IE1234567AB). These examples are useful for structure, but not every documented legacy sample should be treated as a checksum-authoritative test vector. VatDB validates middle-letter legacy variants deterministically by canonical conversion before applying MOD 23. For rare trailing suffix letters outside the modeled lookup table, VatDB uses a not-confidently-invalid pass and relies on VIES for authority.

Source: Irish Revenue VIES guidance (PDF)