One of the things that stands out clearly about this poem is the way in which Hardy has deliberately crafted a poem that is bare of literary devices and presents us with a very straightforward description of a wartime scene and a soldier's musings. You are right in identifying the irony in this poem. However, apart from this, there are no other instances of literary devices. It is as if Hardy has deliberately tried to strip bare his poetry of such techniques to present his message as directly and bluntly as possible. Thus the poem focuses in on the immense irony of the last stanza:
Yes; quaint and curious war is!
You shoot a fellow down
You'd treat, if met where any bar is,
Or help to half a crown.
War is indeed "quaint and curious." It brings about a completely unnatural situation where men are ranged against each other and trying to kill each other, when otherwise they would be happy to buy each other a drink in an inn. Using a very simple style, stripped bare of literary devices, Hardy thus emphasises this message through this poem.