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Not sure what info you have already. Henry had an older brother Thomas born in 1869. Possibly Henry Allen was a brother to Lucia and Louisa was either a sister ot sister in law.
Area - DUBLIN (RC) , Parish/Church/Congregation - ST. AUDOEN
Baptism of THOMAS GULIELMUS GRAHAM of 41 COOK ST on 26 February 1869
NameTHOMAS GULIELMUS GRAHAMDate of Birth17 February 1869Address41 COOK STFatherTHOMAS GRAHAMMotherLUCIA ALLEN
Sponsor 1HENRICUS ALLENSponsor 2LOUISA ALLEN
This may be the death record for Thomas in 1895 (eighth record) note occupation https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1895/05916/4679496.pdf
Found Lucy in the 1901 census http://www.census.nationalarchives.ie/pages/1901/Dublin/Fitzwilliam/Lower_Camden_Street_W__Side/1307032/
Found a 1920 death record for Lucy. Her niece Louisa Allen was the informant Eighth record.
Let me know if you have any questions.