Wouldn't worry too much about the flight plan issue. Only required for crossing/entering the regional airport zones when active, plus the Dublin zone. File over the air 10 minutes before for the regional airport zones (when active). Never had a harsh word yet on that basis. For the Dublin zone, definitely file over via phone/fax/email beforehand.
ATC are generally quite calm, no FIS in Ireland as it applies in the UK in class G. Tell them about yourself 'for information only'. They may be interested in you giving them position reports, they may not, depending on what else is out there.
Technically you should file a flightplan when crossing the FIR from NI to Ireland (as we like to call it), however have done and not done with equal effect. Special Branch as above.
Ah yes Hector, not to be confused with his batty behind the times cousin Millie Bar.
