Had been on the lookout for this for a while, partly because of now knowing the author and also partly because of my general interest in sf and fantasy set in Ireland. Finally picked it up at Worldcon, and, folks, it is good.
Mind you, I spent some time grinding my teeth during the first section, portraying an eccentric astronomer in County Sligo in 1913, because it was completely unrealistic given what was really going on in the Irish scientific community at the time. But I eventually caught myself on. King of Morning, Queen of Day was published in 1991 and presumably written in 1989-90, whereas the only decent book on the Irish scientific community of the early 20th century wasn't published until 1999 and indeed the author didn't even start researching it until late 1991, as I have good reason to know. So it seems unfair to blame ianmcdonald for not having read up on a subject that hadn't been written about yet.
There was, of course, a famous real observatory in County Sligo, built by the Coopers of Markree Castle in Collooney a few miles south of ianmcdonald's fictional Edward Garret Desmond, and I'm sure that he had it in mind. The Coopers had built what was at the time the largest refracting telescope in the world (the largest telescope in the world was the reflecting telecope of the Earls of Rosse at Birr Castle, a hundred miles or so to the south), and discovered an asteroid as well as various other less exciting breakthroughs (it does tend to rain in County Sligo).
I once went to Markree, shortly after I moved back to Belfast in 1991, and with a crowd of fellow enthusiasts searched the ruins (with full permission of the then owners) for what we could find. The telescope is long gone, having been flogged to Hong Kong before the second world war and now located in the Philippines - sinclair_furie, you may wish to verify this). We were at least the third or fourth raiding party to have visited the site, but nonetheless I was lucky enough to find a couple of dozen original pages of Tycho Brahe's designs for his observatory on the island of Hven, published in the early 17th century. I still have them, and one of these days will post transcripts either here or (more likely) to my website.
I also once had an unexpected stay in Sligo town a few years later. My friend K and I were on our way to a stag weekend in Achill Island, and my car broke down just over the border. Ten miles earlier and it would have been back to Enniskillen and a fairly easy journey home; but as it was it was a long wait until we were dragged to Sligo and discovered we were stuck there until Monday. We stayed in the youth hostel, rented bikes, did Yeats' tomb (I still have the t-shirt) and Lissadell. ianmcdonald has put his fictional observatory pretty close to the Drumcliffe cemetery where Yeats lies, so I found it pretty easy to buy into the sense of place.
Indeed, I think this is one of the things I like about his writing, and the writing of many of my favourite authors; the strong sense of place. King of Morning, Queen of Day is set very firmly in three distinct times and places: Sligo (mainly, with a bit of Dublin) in 1913-14; then Dublin (with a little bit of Slieve Gullion and nearby places) in the 1930s; then Dublin again in 1989. All three settings are richly imagined and in fact re-imagined, with an interleaven of creatures breaking through from the Otherworld. The other thing that springs out is that all three central characters are women, indeed three out of four generations; Emily is Jessica's mother, and Jessica is Enye's grandmother. ianmcdonald uses female main characters a lot, and IMHO generally does it pretty well.
The three sections are somewhat different in presentation. The first bit combines diary entries, letters and newspaper cuttings a la Dracula, with the best bits being Emily's engagement with the Otherworld (mapping her father's engagement with the aliens from another planet he imagines to be approaching Sligo). The second section leans (a little self-consciously in places) on Ulysses and Waiting for Godot. The third section seemed to me to be pretty straight narrative, though no doubt there are nuances I missed. I loved the character whose real name was Anne-Marie, but her Ulster accent meant everyone called her Omry. Anyway, I liked it.
One final point of trivia. The middle section has a disparaging reference to Errol Flynn. At the time the Professor of Zoology at the Queen's University of Belfast was his father, Theodore T Flynn. Not a lot of people know that.