Rod, I think from what you have said that the matches are probably valid ones if all the other trees relate to the same Ross family, though there is the outside possibility that you match each of these people through a different line, but they all happen to be connected to the Ross family. That is, you connect to A through Smith, B through Jones, C through Brown etc, but A, B and C are all related to Ross family. With so many Ross descendants matching with you this is perhaps unlikely to be the case.
As davidft says, you will need to compare where you match with those who have uploaded to Gedmatch.
Or compare those who have tested/uploaded to other sites with chromosome browser. I use DNA painter.
https://dnapainter.comI keep one profile for proven matches, another profile where I paint in details of any match, immediately get notified if it matches with any previous one. The advice is to use desktop, but I find it works well on my tablet as well, for convenience.
Hi Margaret
I have just reread all the posts and I think I may have confused the situation. I didn't mean to imply the connection was with my ggg Hannah but rather through her family line, perhaps back to the early 1700s. As you suggest an extra marital relationship could still be a distinct possibility.
Thanks Rod
But I still don't understand why you think it could be Hannah's line. Couldn't it be any of your multiple ancestors where the connection is? For example, have you identified all of your 3G to 5G grandparents? I certainly haven't, and it's not through want of trying - a couple of illegitimacies, some instances where the parish records are incomplete or unavailable, one ancestor from 'Ireland' with no further details available, etc. etc.
If there is even one of these missing, it could be Mr or Miss Ross.
It may be something you will never find out, but it is certainly interesting and over the months/years more matches will appear which might help in your search.
Work as closely as you can with any that reply to your messages, to see if there is a place or time frame that could help, perhaps they have a family story about one of their G uncles who left Scotland for Ireland but came back after a few months because he couldn't find a job, or a story about one of their G aunts who was left pregnant by a travelling Irishman. You never know your luck!
Regards Margaret