I was taken aback by two details of his biography which I must have forgotten. First off, his wife appears to have been a relative of mine, Honora de Burgh, of Portumna in Mayo, whose much older half-brother Thomas was my 5x great-grandfather. Given the time that has passed, their father must by now have tens of thousands of living descendants, so I'm not particularly special in that regard; but it's a pleasing connection of which I was previously unaware.
The second thing that I had forgotten was the local aspect to the story. Sarsfield / Lucan was fatally injured in 1693, at the Battle of Neerwinden (often referred to as the Battle of Landen so as not to be confused with the 1793 Battle of Neerwinden, though the unfortunate hamlet of Neerwinden saw most of the fighting in 1693 as well). Neerwinden is practically the next village to where B lives, and I've driven through it occasionally. The injured Sarsfield was taken to Huy, which is a little further away, where Honora (aged 17, with a toddler and a new baby) was able to join him before he died a few days later.
Honora and her children apparently hung around in Huy for a while, having no other option; but she was rescued from poverty by marrying the even more dashing Duke of Berwick, one of James II's illegitimate children and also the Duke of Marlborough's nephew, and having borne him her third child, she died aged 21 of pneumonia following a miscarriage, still exiled in France. (Her second husband married again, had another ten children, and in 1733 was decapitated by a cannonball during a siege. Apart from that last bit he seems to have been a successful soldier.)
Wikipedia and various Jacobite history websites assert that Sarsfield is buried in St Martin's Church in Huy. A reasonably thorough scan of the internets, however, doesn't give many hints as to the location of a church in Huy with that dedication. There is a Rue St Martin in Huy, but I think it's the wrong side of the river to be in the old town. There is also an architecturally notable Eglise St Martin in Hermalle-sous-Huy, a few km away. On the other hand, Wikipedia records that the Baroque composer Lambert Chaumont was actually based at St Martin's Church in Huy until 1688. So the only answer, really, is to go and look for myself some time. I will report back when I do.