William's WWI Attestation Papers in Canada indicate he was born in Segoe (sic) (Seagoe) Ireland. Dated November 1914 in Halifax.
Do you have death records for John & Priscilla. Scottish records are really quite good with lots of details so you may be able to get the names of both sets of parents.
There are no Justs in the 1901 census in Co. Armagh. Closest are in Co. Antrim and Co. Down, so perhaps that was where John was from, altho' I do find a Thomas Just in Cloncore, Tartaraghan Parish in Griffiths which is certainly in the right area for a marriage to be registered in Lurgan.
A possibility for Priscilla's mother in the 1901 Irish Census:
Residents of a house 32 in Hill (Lurgan Urban, Armagh)
Surname/Forename/Age/Sex/Relation to head/Religion/Birthplace/Occupation/Literacy/Irish Language/Marital Status/Specified Illnesses
Pelan Matilda 64 Female Head of Family Church of Ireland Co Armagh Retired Spirit Dealer Rea and write - Widow - Maginess Margery 74 Female Sister Church of Ireland Co Armagh None Rea and write - Widow - Malcolmson Ann Jane 72 Female Sister Church of Ireland Co Armagh None Rea and write - Widow
Found a birth of a child of Thomas:
JUST, Walter 30 Nov 1869 Tartaraghan Thomas JUST Mary Jane CLARK