If I have an irelia fervent and play a flash and my opponent plays a crumbling sands, do I still get the +1 from choosing irelia despite the spell being countered?

Yes — Irelia still gets the +1 Might.

Her ability triggers the moment she's chosen, which happens when the spell finalizes on the chain — before your opponent can respond with Crumbling Sands. That triggered ability goes on the chain on top of Flash and resolves independently, so even though Flash is countered and does nothing, Irelia's +1 still lands. A countered card is treated as not played, but it doesn't retroactively remove triggers already placed on the chain (Rule 425.1.b; FAQ #10896).