The passenger list for the final voyage of the SS Galway Castle is available at Ancestry courtesy of the National Archives (UK).
The twist is that the passenger list is under a different ship's name, the "Rhodesia." However, the following items on the passenger list all correspond to those of the Galway Castle on her final voyage:
- Official Number (ship's registry number) 132616
- steamship line Union-Castle
- ship's master W. B. Dyer
- tonnage 7988
- departure Plymouth
- destination South Africa
- passenger count 346, including
- passenger Hon. H. Burton (minister of the Crown)
- passenger 2-year-old Miss E. D. Nursey
- passenger Dr. R. A. Buntine, age 49, traveling with teenagers Miss J. Buntine and Miss N. Buntine
SS Galway Castle is said to have departed on 10 September 1918 under the command of captain W. B. Dyer with 346 passengers, ~200 crew members, and ~400 soldiers returning home to South Africa.
The date of departure of the "Rhodesia" is given as 9 September 1918. It is impossible for ship 132616 and master W. B. Dyer to have departed on both 9 September and 10 September, so this must be the same voyage.
Also note that Union-Castle Line did not have a ship named "Rhodesia" in 1918.
Here is the top of the first page of the relevant passenger list at https://www.ancestrylibrary.com/imageviewer/collections/2997/images/40610_B000888-00238 . Note ship name "Rhodesia" and Official Number 132616:
Here is the fifth page of the passenger list showing Hon. H. Burton, KC etc.:
Here is a snippet of the passenger list showing 2-year-old Miss E. D. Nursey, mentioned in a comment:
Here is a snippet showing Dr. R. A. Buntine and two teenage girls who are presumably the daughters mentioned:
Here is the final page of the passenger list showing the passenger count of 346:
For comparison of ship's numbers, here is the top of the first page of the passenger list showing the Galway Castle, Official Number 132616, tonnage 7988, Master W. B. Dyer, sailing from London to South Africa on 29 August 1917:
Here is a letter explaining sailing under a different ship's name, a "camouflage name." This letter is about a different ship, the Agra sailing under the name "Merkara," but the concept is the same. This ship sailed a week or so before the Galway Castle, but the letter is dated a week or so after the Galway Castle was sunk. It's possible that had the Galway Castle not been sunk, a similar letter may have been written for it.
There is plenty more about the Galway Castle at second.wiki, which may be a translation of the German Wikipedia page for Galway Castle. The English page for Galway Castle is about the castle in Ireland; there is no page in English Wikipedia about the ship.
Also here: http://www.bandcstaffregister.com/page38.html
and here: https://forgottenwrecks.maritimearchaeologytrust.org/wrecks-and-sites/explore-the-wrecks/articles/ss-galway-castle
There was a book called The Union-Castle and the War written shortly after the war, with good information about the role Union-Castle's ships played during the war.
Ancestry.com. UK and Ireland, Outward Passenger Lists, 1890-1960 [database on-line]. Provo, UT, USA: Ancestry.com Operations, Inc., 2012.
Original data:Board of Trade: Commercial and Statistical Department and successors: Outwards Passenger Lists. BT27. Records of the Commercial, Companies, Labour, Railways and Statistics Departments. Records of the Board of Trade and of successor and related bodies. The National Archives, Kew, Richmond, Surrey, England.