I wonder....
Tagging more specifically could be useful when making a map and trying to get an accurate sense of the spatial reach of the references in play. But if we can only get as specific as "Belfast," we're going to lose any specificity that would be worth anything.
Would it be worth tagging with an URI where possible and then adding geo coordinates as decimals in the tag?
The other problem that is brought up by this specific context / title is that High Street in 1786 could very well be in a different location than it was in when Heaney wrote it. As well as being different from what Google would tell us today. And these discrepancies could be very interesting, especially the former. But it might make actually getting those decimals difficult.