A basic strategy for 'doing family history' is to start with what you know, and then work outward in small steps, rather than trying to take large leaps. One of our community members has described this tendency to take large leaps as premature connectivitis syndrome:
we don't really know enough yet by which we can well identify a person, yet we want to connect them to a much earlier place in time
When we are stuck and can't go backwards, the solution is to search sideways and forwards in time to gather the information we need to go backwards. There might not be a single record which holds all the clues you need, and you may need to learn how to work with complex evidence -- to do what is called inferential genealogy.
You want to construct a full picture of Robert Ferrier's life in Scotland, and gather all the records from Scotland that you can find, for two reasons -- first, to be able to distinguish his records from those of other people you might find with the same name, and second, to have more clues that might lead you back to the right part of Ireland.
People often didn't move in isolation -- they either moved in groups, or in what is sometimes called chain migration. It is easier to move to a new country when you already know someone in the new place. If we study the rest of the family, including the friends, neighbors, and associates (sometimes abbreviated as the FAN or called a 'cluster'), we can fill in the picture from the movement of the other people associated with the person we are seeking.
Start by making a timeline for Robert, writing down all the information you've found so far, with notes about how you know those facts. Go over the source material again for any clues you've missed. Do the same for any siblings and in-laws. Be on the lookout for any record which might reveal a link between Robert and another person, no matter what kind of record it is or what the association is. Then see what you can find out about his family and associates.
If Robert owned land, look for landowner maps (or tithe maps) that would allow you to map out the neighboring landowners. This technique may yield better results in the US than it would for Scotland, but you won't know until you look. But even if he didn't own land, get a map so you can see where he lived, learn the names of nearby towns, and be able to trace possible migration paths.
In addition to looking for clues about what County in Ireland Robert may have come from, your other goal will be to narrow down the time frame in which Robert, and all his associates, may have come from Ireland. The more you know about Robert's timeline, the better you can identify what records were being kept in his area and which record collections might have something that will help you.
One way to track a family's movement is to look at all the births of their children - are they all in the same place, or in different towns? You may not have civil registrations to go by because of the time period involved, but check other records which might have that information, by using the Scotland Record Selection Table from the FamilySearch wiki.
Local histories of the area where he resides in Scotland can give you a broader picture of how people moved from Ireland to Scotland. You won't have the same records like passenger lists to trace, as people in the US might; it was much easier for people to move from Ireland to Scotland or England. But any technique for studying the migration of groups of people will work, if you concentrate on what clues you can find, instead of dwelling on what you don't have.
Look for case studies from other genealogists who have similar problems -- that could be bloggers whose families lived in the same area as Robert, or formal papers written by genealogists, or anything in between. See what sources and finding aids those other genealogists used. I stumbled upon a book based on a doctoral dissertation that described the immigrant community of one of the towns I'm studying -- which had a section talking about how people moved in groups from the same town in the old country.
If you haven't looked at the offerings of the Family History Societies for the areas where Robert lived, look over their holdings and see what interesting collections you may have missed. Local Record Offices might hold diaries of people who moved from the same town -- and sometimes these diaries mention other people who moved with them.
To reach your eventual goal -- to find his birth record in Ireland -- you'll want to know the name of his townland, which is one of the small jurisdictions in Ireland. Learning more about the nature of the records you hope to find will help you understand the significance of the subtle clues you're looking for. Taking online classes, such as the classes from the Family History Library, or reading finding aids, can help you with research planning.
One final note: you said:
I have searched for any clues using https://www.irishgenealogy.ie/en/, scotlandspeople and Ancestry.co.uk
Do keep a log of what websites you've searched, what searches you've made, and most importantly, how you went about your search. If you take a class and learn about a new search technique, you can look back on your prior searching and see that you haven't tried that yet. It's perfectly okay to write out that you didn't find someone in a particular set of records because the records don't cover the time period you need -- that saves you from running the same fruitless search over and over again.
If you aren't familiar with research plans and research logs, the FamilySearch Wiki has some articles that can help you get started.