Thanks a lot Monica & Ian. Mary Ann Porteous was living in Dumfries on the 1861 census. This latest information means that there is no big problem about Isle of Man, because it is closeby. I was bothered that it might be far away from Dumfries, and couldn't think why she would be out of that area. I had no idea where it was, so thanks again.
I googled a site.... "Place names in Dumfries & Galloway, Scotland."
www.britishplacenames.co.uk/dumfries-and-galloway When you scroll down, you come to Isle of Man, and click it to bring up a map. The map is quite vague though.
As far as I can make out, Isle of Man is just south of the A75, where it turns NW on the eastern outskirts of Dumfries. I think there is a railway line running from Dumfries to Annan, and it is also just south of that. Elsewhere, I saw that the village of Collin is 4 miles from Dumfries and Isle of Man was three miles from Collin, and one mile from Dumfries.
Ian, I think this ties in with what you said.