Your posting has been moved to Ireland general...
Odd that he never stated parents on the marriage cert

Well, let's be sure it's Ireland then....
In 1881 Canterbury, York, N.B.
Thomas says b. 1828 Ireland..( does say ethnic origin Scottish)
In 1881 St James, Charlotte, N. B.
David Slater says born 1825 Ireland with ethnic origin Scottish (no marital status listed...)
in 1881...Living right next to David is a Samuel Slater & family. He says he was born 1819 Ireland...
In 1881, Saint John N.B. There is also a John Slater- c.1823 Ireland..
In 1881, Richmond, Carleton There is an Andrew Slater c.1825 Ireland
and four houses previous is a James Slater c. 1822 Ireland who started out in the U.S. ( children born there)
the next household to that is Mary Greer , (& James) ...c. 1839 Ireland ...children b. Ireland
(* explained later)
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The 1901 Richmond Carlton ( where he has a new young wife ) says born April 15th, 1825 Ireland....immigrated 1840
http://www.automatedgenealogy.com/census/ViewFrame.jsp?id=5971&highlight=47right before them on the census there is a John Slater ( c. 1833 widower) and * Mary Greer c. 1835 (sister, widow, so also a Slater ) both born Ireland ....They may be siblings or cousins...imm. 1860
1901 St James, Charlotte, N. B. This David Slater says he is single but was born Ireland 1830, immigrated 1842
http://www.automatedgenealogy.com/census/ViewFrame.jsp?id=7467&highlight=46~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
I am also wondering if a whole family immigrated to N.B.
Perhaps looking up the marriages for the other Slaters as well, and seeing what information they may have been stated in theirs certs might find some corresponding information to help you get started in Ireland...