As always what i write is open to contradiction but for what its worth...
Hum is correct ANY time you are departing from, arriving into, or flying thru class C you need a flightplan or to put it simply for any flight in controlled airspace. As you leave class C (laterally or vertical) it is then your choice whether you want to close or leave open your flight plan and the protection this gives you and also the responsibility to remember to close it. as far as i am aware the whole class C to G airfield thing and more than 25nm away is poppycock as the only rule is the flight in controlled airspace with or without airports a flightplan is needed rule. However many regionals especially with locally based club aircraft dont bother with the requirement so the aren't overwhelmed with 16 flightplans for the one airplane. As most of the regionals are not connected to the AFTN system if you file a flightplan with the AIS the regional wont know that you have filed a plan until Shannon ATC ring to tell them you are on the way.
Again on this i am open to contradiction but unlike what Hum says if you do cross an international boundary a flight plan is necessary and is indeed compulsory.
And again if you fly at night vfr this can only be done in a control zone with a special vfr clearance and as such a flight plan is necessary.
Hope this is informative and helpful..if not sorry