The first Irish soldiers to join the Russian army were a number of Jacobite officers who left Ireland after the Treaty of Limerick. After first joining the French army they appear to have left for Russia to further their careers. Prominent among these soldiers was Peter Lacy from Limerick. Peter was recruited personally by the Tsar Peter the Great during his famous tour of western Europe in 1700. Lacy became a Field Marshal in the Russian army and was even appointed governor of Riga. He died in 1751. Members of the Brown family, who were also from Limerick, also did well in Russian service.
Perhaps the most famous Irish family to join the Russian army were the O'Rourkes. I have linked the O'Rourkes who entered Russian service to Tiernan Óg O'Rourke, lord of Breifne, who died according to the Annals of the Four Masters in 1468. Count John O'Rourke, the first of the family to make a name for himself in the Tsar's army was the son of a well-to-do farmer from Cloncorike in County Leitrim. After establishing himself as a soldier first in the French army he became a cavalry commander under Empress Elizabeth of Russia. Count O'Rourke had a very colourful career. O'Rourke served the King of Poland for a while, and was part of the Russian army that captured the Prussian capital Berlin, in 1760. Count O'Rourke earned the gratitude of the people of the city for preventing any looting or assaults on the population for the four days he was in command of Berlin. As a result O'Rourke also became a friend of the Prussian king, Frederick the Great, and a number of letters between them have survived. Count O'Rourke wrote an important military instruction book 'A Treatise on the Art of War' which was published in 1778. The Count retired to London where he died in 1786.
Count O'Rourke's nephew, Joseph O'Rourke was a very famous Russian soldier. Born in 1762, Joseph joined the Russian army at a very early age and fought for the Russians in the early Napoleonic wars. When O'Rourke was promoted to Major General in 1810 he was sent with an army under his command to fight the Turks in Serbia. General O'Rourke won important victories over the Turks at Jasika and Varvarino. Major General O'Rourke fought throughout the remainder of the Napoleonic wars and he was brought by Tsar Alexander I to the Congress of Vienna in 1815. He retired from the Russian army in 1819 and died in 1849.
The O'Rourkes continued to thrive in Russia throughout the nineteenth and early twentieth centuries. Unfortunately many of them were killed by the Bolsheviks after the Communist Revolution in 1917. Never-the-less there must still be a few people called O'Rourke living in Russia today.