I believe these baptismal records, found on RootsIreland.ie, are for your ancestors. The Shannons lived in the townland of Tawny in Co. Donegal. Margaret's husband--and Patrick's father--was Hugh Shannon.
Patrick Shannon
Date of Birth:
Date of Baptism: 28-Nov-1818
Address: Tawny
Parish/District: KILMACRENAN
Gender: Male
County Co. Donegal
Denomination: Church Of Ireland
Father: Hugh Shannon
Mother: Margaret
Name: Hugh Shannon
Date of Birth:
Date of Baptism: 06-Aug-1821
Address: Tawny
Parish/District: KILMACRENAN
Gender: Male
County Co. Donegal
Denomination: Church Of Ireland
Father: Hugh Shannon
Mother: Margaret
Occupation:
Name: Rachael Shannon
Date of Birth:
Date of Baptism: 13-Nov-1823
Address: Tawny
Parish/District: KILMACRENAN
Gender: Female C
ounty Co. Donegal
Denomination: Church Of Ireland
Father: Hugh Shannon Mother: Margaret
Name: Catherine Shanan
Date of Birth:
Date of Baptism: 03-Apr-1826
Address: Tawny
Parish/District: KILMACRENAN
Gender: Female
County Co. Donegal
Denomination: Church Of Ireland
Father: Hugh Shanan
Mother: Margaret
Margaret Shannon
Date of Birth:
Date of Baptism: 28-Dec-1828
Address: Tawny
Parish/District: KILMACRENAN
Gender: Female
County Co. Donegal
Denomination: Church Of Ireland
Father: Hugh Shannon Mother: Margaret
Occupation: