I don't think the August vs autumn thing is that big of a deal. We know what the poet is getting at--summer is spent, the prospect of youthful love gone, that sort of thing. I do get the sense that the leaves are about to fall, ie that the end is perceived as being near, not yet arrived. So August works better in that sense, because the poet has had an epiphany about the girl--realizes that he is not to win her. The place he is in is one of poignant resignation to the fact that there is no romantic relationship between them. He isn't looking back in bitterness (that would be a winter season), he is still in the warmth of some sort of relationship with her, but knows it won't go beyond the point it has reached.

I believe the "enchanted way" reference may be to the Grand Canal mentioned above, or to St Stephen's Green (Kavanaugh also mentions Grafton street). There is a strong sense of place in the poem, and that part of Dublin is the place. It is the most romantic part of the city really. Trinity is just north of the Grand Canal and Grafton Street ends at St Stephen's Green, and then as you walk through the green towards Leeson St, on the south end it empties out at least in the direction of the Grand Canal, where Kavanaugh's statue is found between Baggot and Leeson Sts.

This is sort of an aisling poem in English, but it also was written more as a song, than a poem. The angel reference would be a reference to the aisling, rather than the speirbhean, I think. Trifling difference to some, but not if you are trying to evoke the aisling convention. Speirbhean would be more of a political allusion, rather than a romantic literary one, in the Irish sense.

I do think the Queen of Hearts is a mawkishly sentimental throw away line. Kavanaugh, like every other poet, wasn't above or beyond a bad line here and there. It is a bit clumsy for the metre of the verse, I think. But he did well putting the verse to the tune, considering he was a much better poet than songwriter.