There is an outgoing UK passenger list at Find My Past for Wm McGaffin (indexed as McGuffin), 4 McLean St, Partick, Glasgow, plater, age 23 who sailed on the Athenia which departed from Glasgow on 13 Sep 1923 bound for Montreal. There isn't a corresponding Canadian passenger list available.
All from Ancestry:
There is a border crossing document which shows William McGaffin, 23, ship plater, was debarred when trying to cross the border into the US at Niagara Falls, New York in Sep 1923. There is a stamp that says "quota exceeded". Twelve other individuals were debarred for the same reason on the same page. I couldn't find another border crossing document but he obviously made it across the border later.
William McGaffin departed from New York on the Celtic and arrived in Liverpool on 25 Jan 1926. His proposed address was 4 McLean Street, Partick, Glasgow. He was 25 and a machinist. His last permanent residence was USA and his intended future permanent residence was listed as Scotland.
William Thomas McGaffin sailed on the Cameronia which departed from Glasgow and arrived in New York on 17 May 1926. It indicates he was 26 and a machine hand. He said his last permanent residence was in Manchester, USA. The name and address of his nearest relative in the country from where he came was wife: Mrs. Mary McGaffin, 4 McLean Street, Partick, Glasgow. His destination was Manchester, Connecticut.
Mary Elizabeth McGaffin sailed on the Carmania which departed from Belfast and arrived in New York on 29 Mar 1927. The passenger list indicates she was 27 and born in Gilford, Northern Ireland. Her last permanent address was Portadown, Northern Ireland. Her nearest relative in the country from which she came was mother: Mrs. M. McDowell, Bleary, Portadown. She was going to join her husband, Mr. W.T. McGaffin in South Manchester, Connecticut.