Vermont, St. Albans Canadian Border Crossings, 1895-1954
An index that was compiled. And has an entry for Dennis O'Sullivan in 1915 (rather fuzzy image)https://www.familysearch.org/ark:/61903/1:1:QKQM-PS6X
Seems to give when he came to Canada, 10-1903 on SS Carthaginian, landing at Halifax.
All I can see on ancestry!
Name: Gd Sullivan
Departure Date: 17/10/1903
Port of Departure: Liverpool England
Destination Port: Halifax Canada
Ship Name: Carthaginian
Shipping Line: Allan Line
Master: H Gunson
The inward passenger list is not listed on Library and Archives Canada, perhaps it is missing.