The gardens at Mount Stewart are exceptional. So good that they’ve been included on the UNESCO World Cultural Heritage tentative list.
The design is largely the work of 7th Marchioness Edith, Lady Londonderry (although I’m sure she had plenty of help!) The National Trust have some good information on the planning and design of the garden which can be downloaded here.
Here’s a few photos
The Italian Garden
The Spanish Garden
The Shamrock Garden
The Sunken Garden
The Informal Gardens
Down by the Dougie 
Edith Londonderry is included in a new book called “Queen Bees”, about leading society hostesses in the first half of the 20th Century. Will be published this autumn.
Thanks for info. I’m quite curious about her Ark club during WW1 and what influence it had. I did a web search but couldn’t find anything useful on it.