Steve,
The arched words at the top are “In loving memory”. The amount of wording varies in Ireland. You do sometimes see just “The SMITH family” with no other information but perhaps the folk in Randalstown were slightly better off and so put more on their gravestones.
General thoughts about the grave are that the family were quite well off. The average labourer couldn’t afford a gravestone at all and were often buried in unmarked graves. (The family would know where the grave was but there would be no marker). Not only does this plot have a gravestone, it has surrounding metal work which you can see in at least one of the photos. Stonemasons charged by the word, so altogether you have an expensive grave. That in turn tells you the family were probably quite well off. Or someone in America paid for the grave. That was quite common.
Here’s a link to that family in the 1901 census:
https://www.census.nationalarchives.ie/pages/1901/Antrim/Randalstown/Maghereagh/919817/
1911:
https://www.census.nationalarchives.ie/pages/1911/Antrim/Sharvogues/Magheralane/111298/
Around 1901 the family had a total of about 20 acres in Maghereagh, split between 2 separate plots (plots 4c and 6A). The farmhouse was on 6A. 20 acres is not a huge farm by Irish standards but you could support a family reasonably well on that here.
My contact in the parish says that James McLaughlin was also known as O’Laughlin in the plot records. That’s not unusual. The Mc & O’ prefixes on Irish names are often omitted or interchanged.
The names Jane and Jennie/Jenny are also interchangeable here. We have loads of those eg Peggy/Margaret; Sarah/Sally & Agnes/Nancy
You ask when the church and graveyard opened. I have attached pages from “Sweet Drummaul” by Patrick O’Kane which gives a little background. But basically the first church (the one in the photo) was built in 1784, and extended in the 1820s. The graveyard wasn’t opened till 1848. Prior to 1848 families had a choice of half a dozen other graveyards in the area eg Cranfield, Old Drummaul, Milltown and Shane’s Castle.
The church in the gravestone photos is no longer in use. The parish has a splendid new church nearby, built about 20 years ago.
You ask about adding names to gravestones. That certainly happens and you see gravestones with blank space for later additions. In this case the dates are in strict chronological order, so there may have been additions over the years. On the other hand the carving looks remarkably even and that would tend to suggest that all the engraving was done at the same time ie after the last death in 1941. Hard to tell one way or the other.
It’s widely reported in the press here in Ireland that there has been a significant surge in applications for Irish nationality by US citizens since Donald Trump came to power. So you are not alone in your aspiration. As a genealogist it is interesting to note that for 300 years Irish people yearned to go to America but now Americans yearn to emigrate to Ireland. Strange old world at times.