There are many definitions of the word "Jackeen".

It is generally considered to be an unflattering name for a person from Dublin.

I have found the following definitions;

Jack*een"\, n. A drunken, dissolute fellow. [Ireland] --S. C. Hall. Source: Webster's Revised Unabridged Dictionary, © 1996, 1998 MICRA, Inc.

Jackeen - a Dubliner. Dublin was always seen as the most "English" city in Ireland by provincials and this was coined as a term of derision stemming from the English flag, the Union Jack, by adding the diminuitive, -een. Literally, "Little Jack". Derived from "shoneen", a sort of "working-class West Brit".

Jackeen: the opposite of a Culchie

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But mostly, a Jackeen is a person from Dublin.