While Yeats sometimes likes to play games with meter, in "An Irish Airman Foresees His Death" he's pretty straightforward: iambic tetrameter all the way. Wait, don't we mean iambic pentameter? No, actually, we don't, but that's a good point. You see, iambic tetrameter is just like iambic pentameter, except there are four (the prefix tetra- means four) iambs, instead of five.
Iambs? Now what are those? Those, friends, are essentially beats. They're two-syllable pairs that contain an unstressed syllable followed by a stressed syllable. Each iamb makes the sound da-DUM, as in the word "allow." Since we have four of those in these lines, each line should sound like da-DUM da-DUM da-DUM da-DUM. But don't take our word for it. Check out an example:
Nor law nor duty bade me fight. (9)
Voila, piece of cake, right? Oh, wait. What about the first line? It seems kind of fishy. Let's see what we have going on there:
I know that I shall meet my fate.
Well, the last three beats are iambs, but what about that first little bugger? It doesn't really fit the iamb formula. Nope. That, folks, is called a spondee (a pair of two stressed syllables). It is a very emphatic beat. Here, the extra oomph that it gives reflects the speaker's certainty that he will die; it's almost like he's shouting, "I KNOW."
The regularity of the meter is complemented by some very neat organization. We have sixteen lines, organized into groups of four lines (called quatrains). Even though the poem isn't usually printed in groups of four, the rhymes tell us they should be thought of this way. Speaking of rhyme, each quatrain rhymes every other line, so we have a rhyme scheme that looks like this: ABAB CDCD EFEF GHGH, where each letter represents the sound of the end rhyme. And so, line 1 rhymes with line 3, line 2 with line 4, line 5 with line 7, and so on.
Come to think of it, the number four is all over this poem, in various ways: four groups of four lines, and a meter that uses four beats. All the ducks are in a row, as they say. The point of all this regularity and balance (a word the speaker uses, mind you) is to imitate or reflect the speaker's absolute certainty that he will die. The poem rhymes like clockwork; we know how many beats there are going to be, always, in much the same way that the speaker knows he will die. It's predictable and fated—bank on it.