Could be the wrong couple but
In Griffiths Valuation print date 1856 in Haggardstown Parish, Village of Blackrock in section 65 which is the Main Street facing the sea there is a Philip McGuinness with a House and garden at 26 and a John Mathews with a house and yard at 33.
There are several other Mathews on the same street.
As this John Mathews doesn't have even a garden never mind land how does he earn a living?
No proof but oh so tempting.
Of course your John probably wasn't even married by than so he may not even be listed, more likely one of the Mathews on the street is his father.