Is there a name for this technique? (spoiler for Macbeth?)

Sorry if this is the wrong sub, it was the one that seemed to most suit my question but I'll remove it if it's not.

Not a homework question! I'm just wondering as I'm currently studying Macbeth, but won't see my teacher for a few days so came to see if anyone here had any ideas.

In Act 4 Scene 1, one of the apparitions that the witches created/conjured tells Macbeth he:

"shall never vanquish'd be until Great Birnam Wood to high Dunsinane Hill shall come against him."

Now, I already roughly know the ending of Macbeth, and know that the army disguise themselves as a forest and attack him that way.

I was thinking that if the audience already knew this would happen, eg it had been mentioned earlier in the play, this would be dramatic irony. However, I think an audience at the time it had been written wouldn't know the ending, so it would be more foreshadowing of what was to come.

But to a modern audience who would mostly know the ending, would it be dramatic irony? As Shakespeare probably didn't intend for people to know the plot before they saw it performed, so I doubt he intended it to be acknowledging something the audience already knew, and was instead using to foreshadow what happens later.

Thanks :)