If William Banks was born c 1829 then that’s well before the start of statutory birth registration (1864) so you would need to search baptism records for him. If Church of Ireland, then Magheralin records start in 1692. My guide to Church of Ireland records says that rootsireland has some coverage of the Magheralin records (ie not all years). You could search there. Otherwise you need to go to PRONI to view them.
You say that William John Banks was born in Aran in 1851. By that I take it to mean the Aran Islands. (There is also Aranmore Island in Donegal). The Church of Ireland records for the Aran Islands were lost in the 1922 fire, so they have nothing for that period either.
It’s a bit odd for a farmer in Co. Down to have had a son born in Aran. Unless perhaps he worked for the coastguard or lighthouse authorities at one time. Perhaps that’s a mistake? Might be worth checking Magheralin baptism records just in case, especially as you say all his siblings were born in Magheralin.
There are 2 William Banks listed in Griffiths Valuation for Magheralin in 1864. One was in the townland of Drumo & Drumcro where he had plot 13b, a labourer’s cottage. The other was in Feney where he had plot 6a which was an 11 acre farm.
This Banks family in Drumo & Drumcro in 1901, who were Church of Ireland:
http://www.census.nationalarchives.ie/pages/1901/Down/Magheralin/Dromo_…
Administration of the estate of William Banks late of Drumcro County Down Farmer who died 20 January 1889 granted at Belfast to Catherine Banks the Widow
Probate of the Will of Catherine Banks late of Drumo and Drumcro County Down Widow who died 17 November 1915 at Lurgan granted at Belfast to Alfred Waddell J.P. and John Banks, Tailor.
