Just to continue the conversation for other interested parties...
My earliest known Gardner ancestor was James Gardner, born about 1779 in "Ireland" (no other data), believed to have arrived Philadelphia 1811 on the "Bellona" from Londonderry, and settled in Millersburg, Dauphin County, Pennsylvania.
My LivingDNA results as well as the general immigration trend at that time suggest he was from Northern Ireland/Ulster. After reviewing a Scots-Irish research guide, it seemed that the best strategy for narrowing my Irish search was to find concentrations of Gardners in available records on-line. Three locations emerged: Londonderry from the earliest days of the Ulster Plantation in the 1610s; Carrickfergus; and the Lisburn area. Descendants seemed to have spread out from there.
The Public Records Office of N.I. (PRONI) has a published genealogy of Gardners in the Carrickfergus area for which I have requested a copy. That might help me connect dots of descendants in that area and Belfast.
The Derry Gardners start with a William in 1615, and it appears that his son? was mayor by the 1660s. Of course, there was the eventual nobleman whose family ended up in Lancashire from this year as well.
My Y-DNA result from the "Big Y" test at Family Tree DNA is N-BY49960. Going backwards in time, the Y-DNA is N-BY49960>N-FGC14538>N-17113>N-FGC14542>N-L550. That puts me in a group of people who migrated from Finland perhaps 2000 years ago into Norway, then to Scotland with the Vikings, then to Ireland in the 17th century.
I have found another person in the haplogroup who is descended from Gardners in Glasgow in the 17th century, so my Gardner migrant to Ireland would be closely related to this group. Other associated surnames with close matches are Wilson and McLelland.
The N haplogroup is so unusual in Ireland (or the UK in general) that as more people are tested they will stand out clearly as a connection.
Paul Gardner
Shoreview MN USA