You are giving no approximate dates as a help.
Names were not very stable prior to 1900 and could be spelled in different way.
Are sure they are from Dublin?
1901/1911 Irish Census www.census.nationalarchives.ie
1820-1839 Tithes www.genealogy.nationalarchives.ie
There are 40 Casement's in the Northern Ireland - British telephone book, all come with complete addresses and postal codes, you just need to sort out which counties.
Church of Ireland, marriages from 1845-1862:
Robert Casement married Catherine Brown 17 Oct 1850 Saul,
Fa: Thomas Casement Fa: James Brown LDS FHL Film #101315
Alexander Casement married Anne Jane Wigton, Downpatrick
Fa: Robert Casement Fa: James Wigton #101355
Elizabeth Casement married Patrick Dorran 10 oct 1853,
Downpatrick, Down, Ireland
Fa: Robert Casement Fa: Patrick Dorran #101345
John Casement married Jane Osborne Christy, Misc. Ireland
6 Nov. 1849
Fa: Robert Casement Fa: James Christy #101305
Birth records from 1864: www.familysearch.org
Griffith Valuations from 1846-1875 depending on the county.
83 Casement's in Ireland, 40 Casement's in Antrim, some who would probably be of the Sir and gentlemen class as they own their land IN FEE.
25 in Co. Down, 14 Co.Londonderry, 1 in Sligo.
Irish Wills Index:
Philip Casmey, Waterford 1785 Will Transcript
National Archives: T/7425/271
Robert Casmey, Co. Down Sheepland, Parish Dunsford 1834
National Archives, Prerogative Will Book/F/253a
Griffith Valuation, County Down 1863:www.askaboutireland.ie
Robert Casement, Parish Down, Townland Woodgrange, Poor Law Union Downpatrick, tenant of James Magennis
Alexander Casement tenant of Wm. Nesbitt 10d house
Robert Casement, Parish Inch, Townland Magheracranmoney, Poor Law Union Downpatrick, tenant of John W. Maxwell
#13 house, outbuildings and land 19 acres 3 roods 0 perches
Robert Casement, Parish Saul, townland Quoile, Poor Law Union Downpatrick tenant of David S. Ker
4 A land 0.3.0
4B house, outbuildings and land 12.0.25
4Ca land 2.3.25
4D land 1.0.0
8 James Casement tenant Davis S. Ker
house, outbuildings and land 10.0.25
William Casement, Parish of Dunsford, Townland Sheeplandbeg
Poor Law Union Downpatrick, tenant of William N. Wallace
5a house, outbuildings, land 54.2.18
5b house and outbuildings, unoccupied would be tenant of
Francis Casement, Brocklamount, Ballemena, Antrim
Rev. Robert Casement, Harryville, Ballymena
George Casement Esquire, Fenah, Ballymena
Mrs. Christina Casement, Brocklamount, Ballymena
Thomas Casement, Esquire Justice of Peace, Ballec House,
Antrim being in Northern Ireland records can be found onwww.emeraldancestors.com
free to view but pay for records
an Irish History Foundation also with Antrim records
Ballymena Church of Ireland baptisms from 1788-1871
Ballymena RC records from 1836-1899
Co. Down records on the History Foundation unavailable at the moment as being updated.