After prayers, dinner, and fire with the dead souls, people would go out “guising” (e.g. wearing a disguise!) to fool or imitate the Aos Si (history isn’t clear on this distinction), often using the ash from those bonfires to blacken their faces, holding carved-out turnips (pumpkins are native to North America) with candles in them, and going door-to-door singing songs or reciting racy poems in exchange for food. If you didn’t give these weird people dressed up like JV Gods some of your Stouffer’s, they would threaten to do things to your home. So yes, in this version, trick-or-treating essentially started as a kind of f***ed-up blackmailing scheme.
On Christian influences; vengeful soul cakes; slutty cobblers' apprentices:
Move aside, Pagans! Christ’s followers also take credit for some Halloween action, mainly because it falls on the the night before All Hallow's Day, aka All Saints' Day, aka Hallowmas, aka Hallowtide,
aka Hallow Man aka DJ Kevin Bacon -- no... that’s about all of them. On these days, Christians pray for recently departed folk to get up to Heaven, and also pray for saints. People also baked “soul cakes”, which have nothing to do with movies produced by Babyface in 1997, and everything to do with making baked goods for “all christened souls.” Once you made your cake, poor people would come knocking door-to-door to get some, in a process called “souling”. If you didn’t have a cake, the poor people would say passive-aggressive things about you in Latin. So that’s their version of where/how trick-or-treating started.