The key to your argument lies in Gretta. She clearly states in the story that she believes that Michael died for her:
"And what did he die of so young, Gretta? Consumption, was it?"
"I think he died for me," she answered.
ILets break down the details about Gretta: she is from the west of Ireland. Ireland is a country that was usurped by England. The British moved in during the 17th century and worked to eliminate the Irish culture, hoping to remove any threat from the natives by stripping their individuality. However, the west of Ireland - further removed from England and from the ports into which the England invaded the country - retained its culture much longer than other parts of the country. Gretta being from there gives her a more "Irish" identity.
In addition, Gretta is married to Gabriel, who represents the European influence. Gabriel is labelled as a "West Briton" by another party-goer, meaning that he has become English, despite his heritage. He is ridiculed for not wanting to visit his own country and for abandoning his true language. Being the "Briton", Gabriel dominates Gretta as England dominates Ireland. He over-protects, insisting on galoshes, and tries to prevent her from being with "her people."
Thus, Gretta clearly symbolizes the country, and Michael's death indicative of the death of young patriots for their country.