One thing is certain, Gartlan is probably the single most common surname in Carrickmacross, or it was. I'm sure there are many with this surname there today too. Outside of County Monaghan, the name is not quite as common. Stateside in the U.S., it really is not common plus the name has a variation of Gartland with a "d" on the end. There could be other variations too like Gartlin, Gartle, or the like.
I may have to be satisfied with knowing that my 4th great grandfather, James Gartlan, arrived in update New York in the 1820s at the Irish Settlement. He was already married to Hannah Fox and they had three living children (Hugh, Frank, and Mary Ellen) who were born in Ireland and immigrated with them. Mary Ellen married James McLaughlin of Newport, New York, on January 6, 1838. Moving forward in time, I find Mary McLaughlin with her father (James Gartlan) living with she and her family when he was elderly in 1870 at the age of 93 years.
So, I have a lot of information and "proof" of my James Gartlan. I even have the birth record of his son, Hugh, for Ireland.
Will more information show itself for my Gartlan line? I hope to someday retrieve it myself but for now, I wait a while.