No idea about the rules for Northern Ireland (I suspect they are similar to England) but in Ireland (the Republic) wild camping is technically not legal, but in reality as long as you follow the usual politeness rules and ask nicely then pub front gardens (bonus: close to evening's entertainment) and farmer's fields shouldn't be much of a problem. I don't know how easy it would be to walk along, but the Shannon-Erne system neatly bisects Ireland and friends have told me that the pubs along the waterway are absolute hidden gems. Maybe you could thumb a ride on a boat?

And for those who fancy a wee cheat in Scotland, there is the fantastic postbus service to cut a chunk off your hike and get you to some VERY remote places