The Extraordinary Issue One Pound notes with prefixes 01X and 02X
were issued in 1974, and caused a minor panic at the time (as reported then in Irish Numismatics, and other media) as the notes did not contain a metalic security thread (introduced in 1971), and people thought these notes were forgeries.
The notes were a response to a shortage of £1 notes and had been printed in 1963 with non-standard prefixes, 01X and 02X.
Central Bank of Ireland One Pound 1963 01X prefix.
These notes also exist with dark ink date and serial numbers, the mulberry variety.
Not a very good pic on this one, but it does illustrate the difference.
Central Bank of Ireland 1 Pound 11.3.63 01X prefix, Mulberry serial numbers.