Birth certificate.

[josey]

Ah I see. What was the age gap between the younger brother & Margaret? And where was he born? Were any of Margaret's older siblings born in England/Wales - where civil registration started in 1837. as you probably know. When was the first child born - this may give a clue as to when William married.

[Christine53]

This looks like the right one in the Regimental Birth Indices. The certificate should be available from GRO in the usual way.
Gro Regimental Birth Indices Supplement (1761 To 1924)

Margaret Wilson
Ballincollig Ireland

429
p142
line 57

[josey]

Good find Christine; sadly, as I know to my cost, not all army birth certificates give mother's maiden name  but definitely worth a try. If Pearsews can give names of siblings they may be found on those army indices too.

[Pearsews]

Thank you everyone, can't thank you enough for all the help you have given me.
I have ordered the certificate from the GRO with the reference you gave, and I will let you know if Margarets mothers surname is given. (Hope so!)
If it is the next on the list will be William's marriage, if not will have to try one of Margaret's younger siblings.

Thank you all.

[josey]

I am sure I speak for us all in saying 'You're welcome!!'.

Be sure to let us know what the birth certificate says.

Is this the family in 1861 RG 9; Piece: 633; Folio: 96; Page: 3 Portsea Hampshire?
William Wilson   35 gunner Royal Artillery
Mary Wilson   24
Mary A Wilson   7
Eliza J Wilson   5
Allan Wilson   3 [marked as son but name in army baptism register looks like Ellen  ]
Margaret Wilson   2
All born Ireland

All from ancestry - The marriage/baptism register says marriage in Seagoe, Armagh on 17 Sep 1858, but this is a mistranscription from another marriage register which clearly says 17 Sep 1850. This fits in with first child born 1853. Another record states William's father as Mathew Wilson:
Name:   William Wilson
Gender:   Male
Age:   20
Birth Date:   1830
Marriage Date:   17 Sep 1850
Marriage Place:   Seagoe, Arm, Ire
Father:   Mathew Wilson
Spouse:   Mary Kennedy
FHL Film Number:   101318
Reference ID:   2:3PZMTGC

Marriage is not in Seagoe parish register on https://registers.nli.ie/registers so may have been in CoI church or at barracks.

[hallmark]

Difficult to search Registers as Religion has not been given!

[Sinann]

It appears there is a marriage cert so not RC
Mary
http://www.rootschat.com/links/01nbh/
William
http://www.rootschat.com/links/01nbi/
Image not online yet.

[Pearsews]

Hi again,
yes I had seen the records of Williams children on the Royal Artillery sheet. I also noticed a difference between 1850 and 1858 as the marriage and assumed that was a mistake. Hadn't noticed Ellen/Allen though!
Enthused by all the help you lovely people gave me, I found a Civil marriage record of William Wilson and Mary Kenedy (one 'n') in Tamnaficarbet Parish of Seagoe. William is 20 and Mary is 17. Williams occupation is given as a Labourer, and his father a Mathew Wilson, Mary's Father a John Kennedy. Both parents labourers.
So I suppose this means William joined the Royal Artillery afterwards. Unfortunately it seems enlistment details are at the GRO or Kew, and the Royal Artillery depot is relocating and their records cannot be accessed until at least 2020.
Am unable to find either Mathew Wilson or John Kennedy at present.

[Pearsews]

PS. Forgot to say the marriage details were from the 'rootsireland' site.