A couple of notes and observations
I stressed that Ruby not Rebecca was used on the marriage registration to make it easier to search for the information as the original registrations can be viewed as well as transcriptions. Soemtimes, we gain more information that way.
I think you may be dealing with two siblings, John b. about 1841 and Thomas born about 1849. They seem to both originate in the Limavaddy area and have reused the same forenames over and over again. That is a common naming practice in both Ireland and Scotland.
If you look at deaths at Family Search, you will note that the name Thomas Elder is prevalent in the early 1800's so that could also be the name of the father of John and Thomas. Totally speculation but something to keep in mind when looking for information.
Looking at the family of Thomas Elder and Hannah Hunter/Sarah Perry
1901 Census of Ireland
Londonderry
Limavaddy
61 Main St.
Thomas Elder age 52, widower
sons William age 28, Joseph age 24, Thomas P age 24, Fredrick G P age 12, Samuel age 11.
Would appear to have large gap between Thomas and Fredrick so could be children from different mothers.
70 Irish Green St.
John H. Elder, age 13, brother in law, in the household of Robert Kerr
6 Kennaught
Rebecca Elder, age 16, sister in law, in the household of Samuel Wark.
I think this is the Rebecca/Ruby who married John Morrow
Irish Green St.
John Morrow, age 21
1901 Census of Ireland
Antrim
St. Georges Ward, Belfast
161 Donegall Road
John Elder, head of family, age 60, widower
Lizzie Elder, daughter, age 26
Rebecca Elder, daughter, age 33
Matha Campbell, daughter, age 36, widow
John Campbell, grandson, age 13
George Campbell, grandson, age 11
Robert Campbell, grandson, age 8
Wilhelmina, granddaughter, age 5
1911 census, same address has Martha, George, Robert, Wilhelmina, Rebecca Elder. I wasn't able to find John in the census so it doeslook like he moved on between 1901 and 1911. He would be born roughly 1888/89 so may have enlisted in the First World War. You can check for Attestation Papers for Canadian soldiers at
www.collectionscanada.gc.ca/databases/index-e.html