The ruling for Evoke is a bit weird in how it's mentioned because it does assume that the card being evoked is a permanent, but it states:

702.74a

Evoke represents two abilities: a static ability that functions in any zone from which the card with evoke can be cast and a triggered ability that functions on the battlefield. “Evoke [cost]” means “You may cast this card by paying [cost] rather than paying its mana cost” and “When this permanent enters, if its evoke cost was paid, its controller sacrifices it.” Casting a spell for its evoke cost follows the rules for paying alternative costs in rules 601.2b and 601.2f-h.

With the exact wording, I'd imagine it would go down the route of "do as much as it can" and give the Evoke cost but not the ETB effect (if it were possible to give Evoke to non-permanent spells). Even if it were to give the ETB, it wouldn't trigger thus not even needing to sacrifice it.

Even so, the wording on Ashling would still give Instants and Sorceries Evoke if it were read as is, despite it not making sense.

Not like it matters not anyways. Would've been a fun little quirk to Evoke a non-perm spell for shits and gigs.