My country is Kiltartan Cross, My countrymen Kiltartan's poor, No likely end could bring them loss Or leave them happier than before.
Lots to cover here. Let's start with this whole Kiltartan business. We can tell you that it doesn't have anything to do with kilts.
As you've maybe guessed, Kiltartan Cross is a place in Ireland. In fact, it is the name of a barony in western Ireland (a barony is kind of like a county, but smaller).
Kiltartan was home to one Lady Gregory, a very close friend of Yeats' who had this really awesome estate called Coole Park. (It was cool in all senses of the word.)
Yeats spent lots of time at Coole Park, which is why the volume that contains "An Irish Airman" is called The Wild Swans at Coole.
The Irish airman named in the poem's title is Lady Gregory's son, Robert Gregory, who was killed in the First World War. Many of the poems in The Wilde Swans at Coole are about World War I and the way it affected Yeats.
Now that we've gotten that squared away, let's get back to these lines.The speaker is on an anaphora kick, and uses it again in lines 5 and 6 ("My country").
He says his country is "Kiltartan Cross" and that his countrymen are the poor people of Kiltartan. In other words, he feels more Kiltartan than he does Irish.
The local is more important to him than the national. It would be like saying "my country is Texas, my countrymen Texas' poor" instead of "my country is the U.S.A."
The speaker also says that "no likely end" will "bring them loss" or "leave them happier than before." The speaker is saying that no possible outcome of the war ("no likely end") will make the people of Kiltartan lose anything. They will have just as much, and will be just as peachy, as before the war. It sure sounds like nothing can affect the people of Kiltartan.
More than that, this is the speaker's bizarre way of saying that the war is pointless. It won't really change anything for these folks in Ireland.
But that's a puzzling thing to say. The war will definitely affect them. How will the loss of one of its residents, the airman of the title, not "bring them loss"?
Looks like these four lines are a group too, just like the first four (a group of four lines in poetry is called a quatrain). They rhyme CDCD—more on that in "Form and Meter."