Thank you so much for all of the time and effort you've put into this! It's more than I could have expected or hoped for! Such useful information you've uncovered. Will have to have a go at having someone look up PRONI records at some point.
A few of your points have gotten me thinking.

Regarding the 2 William Stewart entries in Griffiths in Moneystaghan Ellis, there’s only the 1 William Stewart. On plot 19a you have William with a farmhouse and just under 10 acres of land. He has two labourer’s cottages on his farm which he sublets. One to William Farrell (19b) and the other to John Egleson (19c). So William is listed once as a tenant and once as a landlord. Most farms had a few labourers cottages. The labourer could pay rent to the farmer in cash or by an agreed number of days labour on the farm each year, or a mix of the two.
You suggested that Jane Jones might be the widowed Jane Stewart reverting to her maiden name. That’s not something I have ever seen in Ireland. A widow normally retains her married name.
I don’t think the Holywood born Susan Stewart is likely to be your family. Bear in mind too that there was no statutory birth registration in Ireland in 1854. The vast majority of church records are not on-line (especially for Presbyterian churches). Roman Catholic records are available on-line and some Protestant churches have their baptisms on-line but the vast majority do not. If Susan was born in Ireland, in 1854, I’d search the records for Ahoghill (where her parents had lived) but none of those records are on-line. You need to go to PRONI to look them up.
I would think that the “gave birth en rote” answer is unlikely. It took less than a day to get from Ahoghill to Dundee. The likelihood of giving birth in the middle of the journey does not seem very high to me.
Thanks for clarifying there weren't 2 separate William Stewarts; that's extremely helpful! I'm still at a loss as to why he was a tenant of someone else if he owned a farm with cottages on it. Why wouldn't he have lived in one of those instead of leasing his house from another person?
I agree it's not a common practice for the women to revert back to her maiden name; however, I've seen it a number of times in Scotland. Sometimes even while still living in the same house as her husband, a wife would have been recorded on census with her maiden name; sometimes when the husband is absent in the census (e.g., just leaves), I've seen them recorded under the maiden name. I've seen it after a death as well. So it was a thought. Though definitely unlikely

I agree that it's likely the record I found isn't the correct Susan Stewart. Will definitely check in Ireland. It does seem with such a short trip it's not likely she gave birth "en route." Not sure of whether the route they would have taken from Ireland to Dundee might have gone through Holywood. There is a 2 year gap between Susan's birth and the next child, Margaret. Could they have originally settled for a while in Holywood before moving on to Dundee? Not critical to my research either way; just more curious than anything.
Thanks also for all the information provided about the travel back and forth between Scotland and Ireland; gives me a better understanding of the historical context which I'm only starting to get into researching. My question was not only about what made them go to Scotland; but what made them go BACK to Scotland. I know with the information I have right now, I have no way of knowing for sure, but my assumption is that they originated in Scotland at some point, particularly with the surname of Stewart. How many generations before John that would have been is uncertain since I've no way of knowing if John was born in Scotland or not, and where in Scotland to even begin looking. The brief look into history I've had would suggest that there was a migration to Ireland from Scotland during the settling of the Ulster plantations.
Again so very much appreciate your help

Jennifer