RootsChat.Com

Unfortunately, FindMyPast is closed to me.

I'm becoming suspicious that the "Rouse" listed in that 1921 Canadian census might actually be Liebert - the name of the paterfamilious seems to be "Alb." - and Liebert is occasionally named as Albert on documents. All the details sound like his: Church of England, born in Ireland, nationality Irish, etc.
He "converted" to Catholicism when he married Eva Alice Rouse (he must have been unaware of the fact that he'd actually been baptised Catholic when he was a sprog).
Attached (if it works) some of the details from the census form. Oh - looking at it again, I think they were in 265 Stantoine Street - oh, St Antoine Street? (maybe St Antoine again after those words).
The attachments didn't work, they were bigger than the 500kb allowed here.

Thank you so much for all this!
Liebert seems to have preferred to live strangely inland for a seaman!
Edward could have been his brother, Frederick Edward Gifford - if there's an age it would be clearer which it was.
There were five of the lads in the Americas at various stages: Liebert (I have no idea where this name came from - perhaps a family name, perhaps from a French branch); Cecil, who became a tragic alcoholic after WWI and died, I think, trying to cross from NY State to get to Liebert; Edward, who went to San Francisco where I've found little about him; Claude, a barrister; Gabriel, an artist, who lived in New York; also their sister Ada, and, briefly, their sisters Helen and Sidney. (I've failed to find Sidney's marriage to Arpad Czira…)
As far as I can track him, Liebert first left home around 16, after an education which included the Metropolitan School of Art and the excellent Dublin school called High School. He worked for the British "Merchant Navy" (British ships sailing around the world collecting and delivering goods).
In 1924 Lancelot Rouse tried to cross into the US, naming Liebert as his father.
In 1928 Liebert and Alice Eva Rouse marry.
(In the 1921 census of Canada, in Montreal, district something like St Autant, the Rouse family includes what looks like Ali(?), Eva and Lancelot - the birthplace of the father is given as Ireland, and everything else about him as Ireland or Irish. I can't read a lot of this, it's very scrawly.)
There are some Rouse births 1879-81 in Ireland listed in the civil records - https://www.irishgenealogy.ie/search/?church-or-civil=all&firstname=&lastname=Rouse+&location=&yearStart=1879&yearEnd=1881&event-birth=1&_day=&month=&mothers-surname=&age-at-death=&relation-0= and there are 205 Rouses in the 1901 census of Ireland http://www.census.nationalarchives.ie/search/results.jsp?census_year=1901&surname=Rouse&exact=&firstname=&county19011911=&county1821=&county1831=&county1841=&county1851=&townland=&ded=&age=&sex=&relationToHead=&religion=&education=&occupation=&marriageStatus=&marriageYears=&childrenBorn=&childrenLiving=&birthplace=&language=&deafdumb=&search=Search&sort=countySort&pageSize=100&houseNumber=&familiesNumber=&malesNumber=&femalesNumber=&maleServNumber=&femaleServNumber=&estChurchNumber=&romanCatNumber=&presbNumberDiv=&protNumber=&parish=&barony=&yearsMarried=&causeOfDeath=&yearOfDeath=&familyId=&ageInMonths=&pager.offset=0
(I tried to save the 1921 census page here but the server spat it back.)

Liebert, (né 1876 Irlande) était beau-père de Lancelot. Ce Liebert etait l'enfant cheri de sa mère; il a travaillé sur les bateaux de 16 ans d'age.