DoB from age 76 in January 1859.

Source:  BB using Cemetery Records at UHF:
The Vow Cemetery
Erected by
Mary Jane and Lettitia Boyd
in memory of
their dear father John Boyd
who died 2nd Jan 9, 1859
aged 76 years
also their dear mother
Elisabeth Boyd
who died 14th Apr 1862
aged 77 years

Aged 77 at death in 1862 according to her gravestone at The Vow.

Source:  BB using Cemetery Records at UHF:
Erected by
Mary Jane and Lettitia Boyd
in memory of
their dear father John Boyd
who died 2nd Jan 9, 1859
aged 76 years
also their dear mother
Elisabeth Boyd
who died 14th Apr 1862
aged 77 years 

DoB from age 80 at death in 1890 - IF I have the right Letitia Boyd.

From the Pilot - Letitia Boyd, Ballymoney, 1890/1Q, aged 80, 1;126.

Letitia Boyd
Ireland Civil Registration Indexes
Name    Letitia Boyd
Event Type    Death
Event Date    Jan - Mar 1890
Event Place    Ballymoney, Ireland
Registration Quarter and Year    Jan - Mar 1890
Registration District    Ballymoney
Age    80
Birth Year (Estimated)    1810
Volume Number    1
Page Number    126

She would be 15 years older than Matilda and still unmarried in 1853, aged 43, and of course, she was still unmarried when her father died in 1859 aged 49.  All sounds very plausible.

D/1890/27/1005/13/91    Letetia    Boyd    27th January 1890    80        Female    Ballymoney(pre-1973 Q4)

Cert shows she was from Drumreagh, a farmers daughter, and she died in the Ballymoney workhouse.  Drumreagh is just a few townlands north of Bellaghbeddy.

The 1901 Census has lots of Boyds in Drumreagh....which could mean she is either a completely different family or she had relations in Drumreagh.

There is also a James Boyd in Drumreagh in the GV:
Griffiths Valuation Record Information
Tenant
Family Name 1    BOYD
Forename 1    JAMES
Landlord
Family Name 2    BENNETT
Forename 2    THOMAS
Location
County    ANTRIM
Barony    DUNLUCE, UPPER
Union    BALLYMONEY
Parish    BALLYMONEY
Townland    DRUMREAGH
Place Name    DRUMREAGH
Place Type    TOWNLAND
Publication Details
Position on Page    33
Printing Date    1861
Act    15&16
Sheet Number    16,17
Map Reference    15

Aged 27 at marriage in 1853.

Name    MATILDA MACKAY
Year of Death    1868
Group Registration ID    N/R
SR District/Reg Area    Ballymoney
Deceased Age at Death    43
Returns Year    1868
Returns Quarter    4
Returns Volume No    16
Returns Page No    118
38; 30th November 1868, Belaghbedy; Matilda Mackay; Female; Married; 43; Farmer's wife; Diabetes, Ill sometime, No Medical Attendant; William Mackay, Present at Death, Belaghbedy.

Probably NOT him:
Registration number    Forename of deceased    Surname of deceased    Date of death    Age at death    Date of birth    Sex of deceased    Registration district    
D/1884/27/1005/10/112    James    Boyd    29th February 1884    82        Male    Ballymoney(pre-1973 Q4)

This man is from Drumreagh with a James Boyd, son, PAD, he is married and a farmer.  I do not think this is him because I doubt my James Boyd had any children.... unless mine married twice.  Drumreagh is a few townlands north of Bellaghbeddy and Maddykeel. 

This is also probably him in the GV:
riffiths Valuation Record Information
Tenant
Family Name 1    BOYD
Forename 1    JAMES
Landlord
Family Name 2    BENNETT
Forename 2    THOMAS
Location
County    ANTRIM
Barony    DUNLUCE, UPPER
Union    BALLYMONEY
Parish    BALLYMONEY
Townland    DRUMREAGH
Place Name    DRUMREAGH
Place Type    TOWNLAND
Publication Details
Position on Page    33
Printing Date    1861
Act    15&16
Sheet Number    16,17
Map Reference    15
And he had a sister, Letitia, just as mine did but I still do not think it is the same man.  Strange that he died in the same year as mine too.........  Hhhmmmm.... interestingly, his wife was also called Margaret!!!!!!!!!!!!!!  She died in 1885, aged 77, from Drumreagh, widow of James Boyd, son, James Boyd of Drumreagh PAD.  BUT, she could NOT marry in 1873 and have a son PAD in 1885!  So, has to be all coincidence.

Oh, one more coincidence....my James married a Margaret Jane Henry and this James lived next door to a James Henry.