My first comment is that there’s a most unusual spread of locations in this tree. I wonder what evidence there is to confirm all the information? You say that Thomas Graham was born in Scotland and died in Queens County, and that he had a son William born in Crumlin, Co Antrim who then ended up in Maghera Co Derry. These places are all hundreds of miles apart. People in Ireland in the 1700s and early 1800s did move around sometimes but mostly they married the girl next door and lived locally. Particularly so, if the family were farmers, as yours appear to have been. Farmers spend years improving their farms, so they don't move around much if they can avoid it, as it means starting all over again. The travelling this family apparently did rings alarm bells for me. I would wonder how accurate some of the information is.
Turning to Robert born in Maghera in 1823, you haven’t said what denomination he was. For births and marriages around that period there are no statutory birth records and you have to rely on church records, where they exist. Not all churches have records for that period and where they do exist they are not all on-line. In the case of Maghera, the Church of Ireland (ie Episcopalian) has records from 1785, the Methodists from 1825. There are 4 Presbyterian churches (Maghera, Culnady, Curran & Swatragh) but none has any records before 1843. The RC records start in 1841. The Church of Ireland records are held in PRONI (the public record office) in Belfast but a personal visit is required to view them. (Or you could hire a researcher to look them up).
There is a William Graham in Crew in the 1831 census. The family was Presbyterian and consisted of 6 males and 3 females:
There was a James & a John Graham in that townland too, so perhaps related. There was also a William in the townland of Craigmore.
The Crew family appear to have been farmers as they are listed in the 1828 tithe applotment records. There’s a William senior and a William junior so 2 separate farms.
Griffiths Valuation of 1859 lists a John Graham farming plot 6 and an Archibald Graham farming plot 19. They were the only 2 Graham farms in the townland by that year. Crew is about half a mile outside Maghera town.
The 1901 census for Crew has just 1 Graham family farming there. I note that a grand-daughter Susan, born c 1875 was born in America:
The nearest Presbyterian church to Crew is probably Maghera. The original building was destroyed by fire in the 1798 United Irishmen’s uprising. The Minister had been one of the local leaders and had to go to America to avoid being hanged. Any records up to that year were lost in the fire. His successor was Rev Charles Kennedy. Unfortunately he wasn’t big on keeping records and there are none for his 40 year career. Records only start when he retired in 1843.
There is a graveyard attached to the church and your ancestors might be buried there. However Presbyterians often used Church of Ireland graveyards so worth checking there. There is also the old graveyard, again used by all denominations.
You might find some useful information on the Maghera Historical Society site:
Most Presbyterians in Co. Londonderry/Derry are descended from Scots who settled there in the 1600s. That’s the typical background, and is what I would think most likely are your family origins. By the 1700s they were starting to leave again so whilst it’s perfectly possible William Graham’s father arrived in the 1700s, he’d be going against the flow so to speak. But it’s not impossible. Scots did arrive then, often soldiers and gardeners. The big estates loved Scots gardeners for some reason. But it would be very unusual for a farmer to have arrived in the 1700s.