Quoting from Postal addresses in the Republic of Ireland - Wikipedia

Each code consists of seven letters and/or digits, with a space after the third character. The first character is always a letter and the second is always a digit. The third character may be either a digit or the letter W, but not any other letter. The remaining four characters may either be letters or digits.

You didn’t specify a regex format, but it would commonly be understood as “[A-Z][0-9][0-9|W] [0-9|A-Z]{4}”

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