I mean that at some point in the GFA it envisages a coming together of interests where there is a majority on the Island of Ireland, for at least closer ties between the two traditions, this as I understand it was how being in the EU helped matters?. Also as I understand it there is now a majority in NI for remaining in the EU, so that's a start! I have to confess I don't know how it would be resolved, but from the history of the island and from what I know from my some of my ancestors, the peaceful resolution is unlikely to be accomplished whilst third parties ( e.g. GB/EU/USA) exercise undue influence.
Given that the majority over the whole Island seems now seems to agreed on something, and that there is provision for self government via Stormont, which the Government in the Republic of Ireland could make overtures to, maybe in some Confederation of Ireland (if Confederation is the right term?) formation, now would be the time to do it? If direct rule is restored that window of opportunity, especially if Brexit proceeds to some sort of conclusion (although admittedly that is hard to imagine at the moment) is likely to be closed, perhaps for the foreseeable future.
I take your point about GB's responsibility but if there is ever going to be resolution it will in the end have to be sorted by the Irish themselves.