Possibly because across the EU the 'Habitual Residence' rule has been implemented without any real understanding. I believe that even from desk to desk - let alone dept to dept - of any govt agency it's been implemented differently, so there's always cause for appeal. The official guide here is as follows (note the emphasis on 'or common travel area' - ummm should appeal that):
What is habitual residence?
Habitual residence means you have a proven close link to Ireland. Proving you are habitually resident relies heavily on fact. If you have lived in Ireland all your life, you will probably have no difficulty showing that you satisfy the factors which indicate habitual residence – see list below.
The term habitually resident is not defined in either Irish or EC law. However, the following 5 factors, which have been set down in Irish and European law, will be examined to find out if you are habitually resident in Ireland:
Length and continuity of residence in Ireland or other parts of the Common Travel Area
Length and purpose of any absence from Ireland or the Common Travel Area
Nature and pattern of employment
Your main centre of interest
Your future intentions to live in Ireland as it appears from the evidence
http://www.citizensin...assistance_in_ireland