Hi Jodie
This one's age is corresponds to your information:
1871 - Camrose Pembrokeshire Wales  RD :HaverfordwestED:28
REF: Class: RG10; Piece: 5530; Folio: 10; Page: 13
Keeston Village
Head: Thomas Devereaux, widower, 56, Farm Labourer
Dau: Mary DEVEREAUX unm, 33, General Servant
Dau: Eliza DEVEREAUX. 11, Same (?)
Dau: Martha DEVEREAUX 11ms
All born Camrose, Pemb.I don''t think the relationships are correct here - On the 1881 Census this Thomas is listed as a fatherinlaw age 65, living with Head: William SMITH and his wife Martha age 27. And also in the house a niece, 
Martha DEVEREAUX age 10. So I think in 1871, the 2 children Eliza & Martha are Thomas' grandaughters.
view 1881  here: http://www.familysearch.org/Eng/Search/frameset_search.asp?PAGE=census/search_census.aspGoing back to 1861, seems to confirm this:
1861 - Camrose Pembrokeshire Wales  Ecclesiastical parish:St David 
RD :Haverfordwest ED:28
REF: Class: RG9; Piece: 4166; Folio: 20; Page: 14
Keeston
Head: Thomas DEVEREAUX, 46, Highroad Labourer, b Camrose Pemb.
Wife: Martha DEVEREAUX 47, b Rock (Roch?) Pemb
Son: Stephen DEVEREAUX 14, Farm Servant
Son: George DEVEREAUX 10, b Camrose Pemb.
Dau: Martha DEVERAUX 7, b Camrose Pemb.
Gr-Dau: Eliza  DEVEREAUX 1, b Camrose Pemb.In 1871  is also this young Martha whose age returns a birth year of around 1867, who  can't be discounted as 'your' Martha given how ages are often 'out' on BDM's and Census. 
1871 Boulston Pembrokeshire Wales RD :Haverfordwest   ED:20
REF: Class: RG10; Piece: 5528; Folio: 53; Page: 2
Piskin
Head: Thomas DEVEREAUX , 28, agricultural Labourer, b Camrose Pemb.
Wife: Margret DEVEREAUX  37, b Llanwnda, Pemb
Son: WIlliam DEVEREAUX 8, b Llanwnda, Pemb
Dau: Martha DEVEREAUX 4, b Wallace Castle, Pemb
Very possibly this  younger Thomas is a son of the Thomas the widower of Martha as per 1861, 1871.
In 1891, the Martha I listed first (age 11 months in 1871 Census) appears to be here:
Steynton Pembrokeshire Wales 
Town:Milford Haven 
RD :Haverfordwest  Sub RD :Milford  ED2 
REF: Class: RG12; Piece: 4533; Folio 16; Page 5
22 Charles Street
Servant: Martha DEVEREAUX, unm, 20, General Servant, b Keyston Pemb.in Household of William COLE  widower age 29,  Mason, b Pemb. and his relative Annie BISHOP age 14 b Middlesex London.
I see your Martha in 1901 Census in England, married to Walter BALDWIN where as you say, she is listed as born Haverfordwest around 1869.
Probably need to get her father's name - and her age -  off marriage record now, and try getting back that way to confirm which one in the earlier Census is her?
Cheers  

AMBLY