If you have marriage certs for the Healy sisters, is their father (+ mother?) not mentioned? Even one parental name would help.
Also, where did you get their birth dates, assuming you don't have the birth certs?
Another lead would be the Army records for their brother John. I think these would have the best info.
I had a look on ancestry for the births of the Healy girls, but without knowing a county, it would be hard to pinpoint them; however, there is this one for an ESTHER GERTRUDE HEALY, born Dublin South, in the period Aprl-May-June 1915 (1 year out) which might be of interest:
Ireland, Civil Registration Births Index, 1864-1958
Name: Esther Gertrude Healy
Date of Registration: Apr-May-Jun 1915
Registration district: Dublin South
Birth Country: Ireland
FHL Film Number: 101076