Thanks Boo.
As you will see in my post above, the first child of Catherine Dunn & Mr Hogan was bp a Hogan, but registered as a Dunn. His death was registered in the same quarter (Dec 1855.) The next child was registered as Hogan, mmn Dunn in 1856, so theoretically (ha ha) there should have been a marriage between the two births.
I know nothing about Julia Dunn prior to the 1860 birth of her first child; however, I think Michael McDonough was probably in the city in 1851 census, possibly with two siblings, their widowed mother and other McDonough relatives. So, my guess is that Northumberland was the most likely venue for their marriage, if indeed they bothered.
In 1861 this couple shared a multi-occupancy residence with a third couple (McDonoughs) and their month old child. The child was registered as McDonough, mmn Parker. Later in 1861 this Mr McDonogh and Miss Parker married in Middlesbrough, before moving to Scotland.
Yes, mangled and morphing names (both forenames & surnames) and no knowledge of years of birth doesn't make it very easy, does it?

Without a specific parish in Ireland, Irish research is tricky. However, over the years, NOTHING has ever come up for either pairing in any index of Irish records, or UK for that matter.
I have now reached the conclusion that the parish priests were probably more focussed on baptisms of children rather than on the marriages of the parents.