Those that I fight I do not hate Those that I guard I do not love; (3-4)
The speaker is a strange kind of soldier, one that doesn't seem too patriotic. He doesn't "love" the people he guards—i.e., the British citizens back home. He feels Irish, not British.
My country is Kiltartan Cross, My countrymen Kiltartan's poor, (5-6)
The speaker is patriotic all right, but only when it comes to a really small slice of Ireland: Kiltartan Cross. The people he really cares about, and the ones he really wants to "guard" are the poor back home, not all those nameless people in England and elsewhere that he's never met.
Nor law, nor duty bade me fight, Nor public man, nor cheering crowds (9-10)
Public officials and cheering crowds are usually able to stir up some patriotism. That doesn't seem to be the case here. They don't make a dent in our speaker, who does things for his own reason. He may not even be that patriotic of a dude, as is hinted in the lines right after these.
A lonely impulse of delight Drove to this tumult in the clouds; (11-12)
The speaker didn't join the air force on account of some patriotic feeling. He was driven by his own unique, internal impulse.
The years to come seemed waste of breath, A waste of breath the years behind In balance with this life, this death (14-16)
The ultimate patriotic statement is here: dying for one's country in war makes the rest of one's life seem like a waste of breath. Hmm, this is strange, given the weird absence of patriotism from many of the speaker's other remarks. Perhaps this is ironic, or perhaps the speaker has just changed his mind.