Complex sentence: English-speaking Catholics first assembled in Montreal at the Notre-Dame-de-Bon-Secours church in Old Montreal in 1817, however their numbers were swelled by the massive arrival of Irish immigrants.
Split into simpler sentences: English-speaking Catholics first assembled in Montreal at the Notre-Dame-de-Bon-Secours church in Old Montreal in 1817. Their numbers were swelled by the massive arrival of Irish immigrants.