Walter Grindlay Simpson married Anne Fitzgerald Mackay on 13 January 1881 in the Manse, Banchory-Devenick, Kincardineshire. The marriage was performed by William Paul DD, Minister of Banchory-Devenick, which gives the lie to the suggestion that it was an irregular marriage.
Her parents were Alexander Mackay, architect, and Isabella Catherine Thomson. According to the 1911 Census, Anne was aged 53 and born in Thurso, Caithness, so she would have been born about 1857/8. I had been unable to find a birth until now, because she does not seem to have used the name Rose on her marriage of in the census, but I now see a birth of Rose Anne Mackay on 31 August 1856 in Thurso to Alexander Mackay and Isabella Thomson, who were married in Wick, Caithness on 7 November 1854.
Normally I don't research the parents and ancestors of people who marry into my family, but I have had a brief look just now. However Alexander M(a)ckay is not listed in the Dictionary of Scottish Architects, and I have failed to find the family in any census so far. Anne gave her address in 1881 as Kintore, Aberdeenshire.
I wondered why they chose to marry in Banchory-Devenick, and from the 1881 census I see that Margaret, wife of William Paul DD, was born in Caithness. However her surname was Smith, so there is no immediately obvious connection. Her father was William Smith, Minister of Bower, Caithness, and I think that her mother was Ann Longmire Sinclair.
So I can only confirm that, according to Anne's marriage certificate, her father was indeed Alexander Mackay, architect, but at the moment I cannot tell you anything more about him.