The CSO started including síneadh fadas and other diacritical marks that year, and the baby names reflect this. Previously
Seán and
Sean were included as
Sean. Irish people can be casual about fadas anyway in everyday use.
You'll find this information note on the CSO baby names page at the bottom.
“Information note
The CSO’s Irish Babies' Names 2023 publication includes the síneadh fada and other diacritics since it was first introduced for names registered in 2018. This change has resulted in a break in the series, with new entries created for names with the síneadh fada and other diacritics.
This has an impact in the order of the popularity of some forenames. For example
Sean (without the síneadh fada) has always been in the top 100 most popular names for boys.
Seán with the síneadh fada is now included in the data and both spellings are treated as 2 separate names since 2018, thus
Seán has a rank of 14 while
Sean has fallen to rank 124 in 2023."