We think this is her:
Ellen Dwyer Age: 18 born c 1861
Ship Name: Devon
Port of Departure: London, England
Port of Arrival: Brisbane Arrival Date: 5 Feb 1879
Mmm... she is a puzzle - so Ellen didn't travel with any family
From
http://www.rootschat.com/links/014v8/ Index to Registers of Immigrant Ships' Arrivals 1848-1912there are also these entries
DALTON Ellen age 16 Ship Ironside arrival 21 May 1878
DWYER Ellen age 17 ship Carnatic arrival 5 Feb 1879 also a Johanna age 21
DWYER Ellen age 17 Ship Zamora arrival 26 Apr 1877
there's also this birth entry on FreeBMD in Merthyr Tydfil but a little earlier
Ellen DWYER Sep 1860 Merthyr Tydfil
From Queensland Marriages, Births etc…..
http://www.rootschat.com/links/014vd/ Marriage 1880 Michael McMahon Ellen Dwyer ·
Birth 1880 Maria Agnes McMahon father Michael mother Ellen Dwyer
Birth 1882 Michael Beaconsfield McMahon f Michael mother Ellen Dwyer
Death 1884 Ellen McMahon father John Dalton mother Catherine Finn
No birth entry on FreeBMD for an Ellen DALTON in Merthyr Tydfil that I can see
only these entries
Ellen DALTON
Jun 1861 Chorley (Lancashire)
Ellen Kate DALTON
Dec 1861 Wandsworth (London)
Ellen DALTON
Dec 1861 Radford (Nottinghamshire)
I also can’t see a marriage on FreeBMD for a
John DALTON to Catherine Finn/Faen/Dwyer
or
John DWYER to Catherine Finn/Faen

There’s a picture of
Ellen Dwyer b1862 d1884 immediate family members including Michael & Micheal Beaconsfield, John
unfortunately can't view it on Mundia and you need a subscription to ancestry to look at further details
These are the only two Ellen Dwyer I can find on 1871 census
One with parents John & Catherine, the other with mother Catherine
Ellen born c1856 living in Yorkshire parents John & Catherine
https://familysearch.org/pal:/MM9.1.1/KZG5-8CVEllen born c1866 Glamorgan living Merthyr Tydfil
https://familysearch.org/pal:/MM9.1.1/V51P-K8KCatherine’s mother Margaret McCARTHY is with them – so I think she can be
ruled out