See if she is any of these (the younger ones)
http://www.census.nationalarchives.ie/search/results.jsp?census_year=1911&surname=Dillon&exact=&firstname=Mary&county19011911=Mayo&county1821=&county1831=&county1841=&county1851=&townland=&ded=&age=&sex=F&relationToHead=&religion=&education=&occupation=&marriageStatus=&marriageYears=&childrenBorn=&childrenLiving=&birthplace=&nativeCountry=&language=&deafdumb=&houseNumber=&familiesNumber=&malesNumber=&femalesNumber=&maleServNumber=&femaleServNumber=&estChurchNumber=&romanCatNumber=&presbNumberDiv=&protNumber=&parish=&barony=&yearsMarried=&causeOfDeath=&yearOfDeath=&familyId=&ageInMonths=&search=Search&sort=&pageSize=50