irelia is on abandonded hall, and plays flash, does she get +2 OR IT resolves at base?

No — Irelia does not get the +2. Flash resolves first, moving her to base; only after that does Abandoned Hall's "when a player plays a spell" trigger go on the Chain. By the time it needs to choose "a unit you control here," Irelia is at your base, so she isn't a legal choice (Rule 352.4.b / FAQ #11451, #11605).

That means neither part of the +2 happens: Abandoned Hall's +1 can't target her, and her own "when you choose me" trigger can't fire because she was never chosen. If you have another friendly unit still at Abandoned Hall, that unit could receive the +1 instead.