It turns out that many of the Kate O'Briens that I have read have not in fact been Virago Modern Classics, but I have read Land of Spices and The Ante-Room.
The first is a wonderful tale of two girls coming of age in a convent school, and has been published by Virago four times. I absolutely loved this book and recommend it to anyone who has enjoyed other VMC convent/school books such as Frost in May or Olivia.
The latter is a portrait of an Irish Catholic family in the late 19th century and is an equally exceptional and involving read. If you haven't read any Kate O'Brien before then I would recommend either of these. It has also been published four times by Virago with a similar sequence of editions.
So that leaves me with That Lady, Without my cloak, and The last of summer to look forward too.