Where Once Irish-Town...

 The first house at 427-429 Lisgar was a small wooden double built in 1875 or shortly before, when this neighbourhood was the heart of Ottawa's Irish Catholic community — St. Patrick's Basilica was, and still is, less than 100 yards to the north. 

The present structure is likely pre-1900. The halves have themselves divided further, hence the four mailboxes (two hidden behind the tree.) The strange wooden structure in the middle of the facade apparently houses a spiral staircase affording outside access to the upper apartments.