My dads FTDNA Y67 shows no Harrisons at any level. Many of the better matches are Welsh Patronymic Surnames. This leads me to think the line is recently (200-250 yrs ago) Welsh, and Harrison was the patronymic surname which converted to a fixed surname. Does this sound right?
I know it can often mean a Non Paternal Event. But the fact that the closest matching surnames and most frequent surnames are Lewis, Davis, Davies, Rogers, Phillips, Williams, and more, and some of those people have Most Distant Known Ancestors in Wales, seems to suggest its plausible my family are very recent fixed surname uptakers and used patronymic surnames as late as c1800.
I can test distant cousins to confirm I have the right family tree, and if so, I have papertrail records that take me back with certainty to my gg grandfather in 1828, aged c18ish. On that 1828 record he said he was born in dublin (and he was in dublin at the time, and returned there in his old age) and in later records named his father (my ggg grandfather). But I've not been able to find his birth c1809-1812, or link him to the man he names as his father (who is also named as being a Harrison, and who I can predict was born c1785).
My current prediction is my ggg grandfather (b abt 1785) was indeed a harrison, but got his surname via the patronymic way - ie his own fathers forename was Henry or Harry, and he became a Harrison. But it just so happened that his own son was born at about the time surnames changed from patronymic to fixed in Wales (c1809-1812) and he decided to give his son (my gg grandfather) the surname Harrison too, instead of his own forename as the sons surname. Make sense?
I feel overwhelmed having to look at a whole new country, particularly Wales. Daunting. As if England, Ireland and Scotland didn't already take me years to become a solid novice at researching!