if this is the Mary Gibby living in Narberth, her age varies on the census entries, but gives a date of birth of 1814 or 1817. She was of full age at her marriage on 21 March 1839, so born rather before 1820. I don't think her mother was Margaret Meyler as she married William Harries 3 May 1814
I haven't found a baptism but there is a possible family in Fishguard. There's a very useful website for Welsh Mariners
http://www.welshmariners.org.uk/and there's an entry on that site for a David Harries born 1781 with mention of a daughter Amelia
1851 census
Gilvach, Fishguard HO107/2480 folio 139 pg 14
David Harries head mar 64 superannuated mariner b. Fishguard
Martha wife 63 b. Llanchare
Amelia dau 20 b. Fishguard
1841 Gilvach Goch HO107/1446/7 folio 7 pg 6
Martha Harries 55
James 15
Amelia 10
Possible marriage in Llanychaer David Harry of Fishguard & Martha William of Llanychaer 1 Nov 1808
or
David Harries, mariner, & Martha David 13 Feb 1814 in Brawdy
David apparently died in 1860 according to Welsh Mariners site, so this reg
David Harries aged 74 dec qtr 1860 Haverfordwest vol 11a pg 415