Princess Irene, Duchess of Aosta
Princess Irene of Greece and Denmark( 13 February 1904 – 15 April 1974) was the fifth child and second daughter of Constantine I of Greece and his wife, the former Princess Sophie of Prussia.
She was a member of the royal families of Greece and Italy.
From 1941 to 1943 she was also officially Queen Consort of Croatia.
Princess Maria of Greece and Denmark
Princess Maria of Greece and Denmark  (3 March 1876 – 14 December 1940) was a daughter of King George I of Greece and his wife Grand Duchess Olga Constantinovna of Russia.
She was a sister of King Constantine I of Greece and a first cousin of Tsar Nicholas II of Russia.
Born as a princess of Greece and Denmark, she was educated in Athens by private tutors.
Her father instilled in her a great love for Greece and throughout her life, she remained a fervent patriot.
She married Grand Duke George Mikhailovich of Russia, her first cousin once removed, who courted her for five years.
The wedding took place in 1900 in Corfu.
The couple settled in St. Petersburg and they had two daughters: Princesses Nina (1901 -1974) and Xenia of Russia (1903 -1965).
Grand Duke George Mikhailovich had a house built for her in Crimea and he was a devoted father and husband, but the marriage was unhappy.
Princess Maria, known upon her marriage as Grand Duchess Maria Georgievna of Russia, neither get to love her husband nor her adopted country.
Longing for her native Greece, she never adapted to live in Russia.
Over the years, she became estranged from her husband taking any opportunity to spend time abroad.
At the outbreak of World War I, she was vacationing in England with her daughters and chose not to return to Russia.
During the conflict, she was a patron of three military hospitals in Harrogate, which she financed generously.
Her husband was trapped in Russia after the revolution and he was shot by the Bolsheviks with several other Romanov relatives in January 1919.
In her widowhood, deprived of her Russian income, Princess Maria faced serious financial difficulties.
She returned to live in Greece with her daughters in 1920.
There, she began a relationship with Admiral Perikles Ioannidis, who had been the commander of the ship that she took at her return to Athens.
They were married in 1922.
The proclamation of the Second Hellenic Republic in 1924, sent her into exile.
The princess returned to live in Britain for a couple of years and she settled in Rome in 1926 with her second husband.
She lived in Italy for over a decade until the outbreak of the Greco-Italian War in 1940 forced her to return to her native Greece.
She was in poor health and was cared for by her nephew King Paul of Greece and his wife Frederica.
She died, as the Greek royal family was about to leave for exile.
She left a book of memoirs, published posthumously by her grandchildren with the title "A Romanov Diary".
Princess Anastasia of Greece and Denmark
Princess Anastasia of Greece and Denmark ("née" Nonie May Stewart; 20 January 1878 – 29 August 1923) was an American-born heiress and member of the Greek Royal Family.
She was married to Prince Christopher of Greece and Denmark, the youngest child of King George I of Greece and his consort, Grand Duchess Olga Constantinovna of Russia, but was created a princess "suo jure".
Olga Constantinovna of Russia
Olga Constantinovna of Russia (18 June 1926) was Queen consort of the Hellenes as the wife of King George I.
She was briefly the regent of Greece in 1920.
A member of the Romanov dynasty
, she was the daughter of Grand Duke Constantine Nikolaievich and his wife, Princess Alexandra of Saxe-Altenburg.
She spent her childhood in Saint Petersburg, Poland and the Crimea, and married King George I of Greece in 1867 at the age of sixteen.
At first, she felt ill at ease in the Kingdom of Greece, but she quickly became involved in social and charitable work.
She founded hospitals and schools, but her attempt to promote a new, more accessible, Greek translation of the Gospels sparked riots by religious conservatives.
On the assassination of her husband in 1913, Olga returned to Russia.
When the First World War broke out, she set up a military hospital in Pavlovsk Palace, which belonged to her brother.
She was trapped in the palace after the Russian Revolution of 1917, until the Danish embassy intervened, allowing her to escape to Switzerland.
Olga could not return to Greece as her son, King Constantine I, had been deposed.
In October 1920, she returned to Athens on the fatal illness of her grandson, King Alexander.
After his death, she was appointed regent until the restoration of Constantine I the following month.
After the defeat of the Greeks in the Greco-Turkish War of 1919–22
the Greek royal family were again exiled and Olga spent the last years of her life in the United Kingdom, France and Italy.
Princess Maria-Olympia of Greece and Denmark
Princess Maria- Olympia of Greece and Denmark( born 25 July 1996) is an American fashion model, socialite and member of the Greek royal family.
She is the oldest child and only daughter of Pavlos, Crown Prince of Greece and his wife, Marie- Chantal Miller.
Her paternal grandparents are Constantine II of Greece and Anne- Marie of Denmark, who were the last King and Queen of the Hellenes, while her maternal grandfather is duty free entrepreneur Robert Warren Miller.
She is currently sixth in the line of succession to the former throne of Greece, after her father and brothers.
Princess Elizabeth of Greece and Denmark
Princess Elisabeth of Greece and Denmark( 24 May 1904 – 11 January 1955) was the middle daughter of Prince Nicholas of Greece and Grand Duchess Elena Vladimirovna of Russia.
Princess Maria Pia of Bourbon-Parma
Princess Maria Pia of Bourbon- Parma( née Princess Maria Pia of Savoy; born 24 September 1934) is the eldest daughter of Umberto II of Italy and Marie- José of Belgium.
She is the older sister of Princess Maria Gabriella of Savoy, Vittorio Emanuele, Prince of Naples, and Princess Maria Beatrice of Savoy.
Princess Theodora of Greece and Denmark (1906–1969)
Princess Theodora of Greece and Denmark( 30 May 1906 – 16 October 1969) was the second child and daughter of Prince Andrew of Greece and Denmark and Princess Alice of Battenberg, and the second eldest sister of Prince Philip, Duke of Edinburgh.
Princess Xenia Georgievna of Russia
Princess Xenia Georgievna of Russia( 22 August 1903, Mikhailovskoe, Russia – 17 September 1965, Glen Cove, New York) was the younger daughter of Grand Duke George Mihailovich of Russia and Princess Maria Georgievna of Greece and Denmark.
She was a granddaughter of George I of Greece and first cousin of the brothers King Alexander I of Greece, King George II of Greece, King Paul I of Greece and their sister Queen Helen, Queen Mother of Romania and also of Prince Philip, Duke of Edinburgh.
Princess Irene of Greece and Denmark
Princess Irene of Greece and Denmark( born 11 May 1942) is the youngest child and second daughter of King Paul of Greece and his wife Princess Frederika of Hanover.
She is the younger sister of Queen Sofía of Spain and of the deposed King Constantine II of Greece and maternal aunt of Felipe VI of Spain.

Question: Who is Princess Maria Of Greece And Denmark's maternal grandmother?
Answer: Princess Alexandra of Saxe-Altenburg