Hello!
I am a Sherbrookoise with franco-irish roots so I can offer some more details that may be of interest or even helpful!

Quebec's Eastern Townships were first settled by the United Empire Loyalists, so it was English country at first.
St Peter's and its wee cemetery are both still intact. Funny, I never heard it referred to as the Irish Church; I just assumed anyone who spoke english wound up using it.
Sherbrooke has a 'town' attached to it, to its west known as Lennoxville, where the anglophones tended to make their homes. It is home to the prestigious institution Bishop's University-which houses the ETRC The Eastern Townships Resource Centre - with a special focus on the English-speaking community.
www.etrc.caOn there you will find many, many links that can help you. The staff are fantastic too. There is also the eastern townships heritage webmagazine that always has wonderful picures including bridges!
www.townshipsheritage.com(Be sure to visit Marilyn Simmons' genealogy website.)
As far as bridges go- your grandfather would have been kept busy in Sherbrooke alone, as there are two major rivers there - the St Francis and the Magog. (The latter was a source of hydroelectric power). There are three bridges: the King, Wolf and Dufferin Streets bridges. See pictures of all three at the ETRC site.
And your military hunch is on the money. We had the first bilingual regiment in the country in the Sherbrooke Hussars.
http://www.army.forces.gc.ca/sherh/qg-hq/historique-history-eng.aspx?action=view&id=1gm-ww1(Be sure to read the story about 'the bomb'- I have been on it!)
That ought to keep you busy for a while, but feel free to pm me for local insights any time. Or if you need help.
Glad to blab about home-just grab a cuppa first!
Nathalie (Frirish)