Breifly, western Ukraine was part of the Austrian Empire in 1915; you can see the extent of Ukranian ethnic groups in the Austro-Hunagrian empire in this ethnographic map. In 1772 the Polish-Lithuanian Commonwealth had been partitioned and the region called Galicia was transferred to the Austrian Empire. The "Kingdom of Galicia" became one of the constituent realms of the Austrian Empire, along with the Kingdom of Hungary, Bohemia, the various possessions on the Balkans, and of course the Empire's Austrian heartland.

As an outsider it seems rather weird that someone would be discharged from the Austrian army around 1915 and then move to Canada, all while there's a major war going on at the same time. Do you have any further details on his journey?

The vast majority of Ukrainians in Canada descend from Western Ukrainian immigrants who left the Crownland of Galicia (this is modern day Lviv, Ivano-Frankivsk, Ternopil oblasts), also the Duchy of Bukovina (Chernivitsi oblast).

Most left between the early 1890s up until 1914.

The fact that your great grandfather served in the army means he came from a moderately wealthy family in the area, indeed; only 10% of Galicians were nourished enough to be conscripted. Generally, 10 years were served in the army so your grandfather probably served in the 1890s-1900s.

Also, Western Ukraine wasn't "briefly" part of Austria like the poster above stated, they were part of the Austrian empire and later A-H empire for almost 150 years...