A little more information about travel between Ireland and Scotland over the centuries.
According to an Ulster Scots Agency publication about the Reformed Presbyterian Church, when there was a shortage of Presbyterian Ministers in Ireland in the 1600s, “as many as 500 members of Rev Livingston’s former congregation at Killinchy, Co Down would sail to Stranraer on a Sunday, worship with him there, and sail home again that evening.” And that was in the days of sailing ships. Clearly people went back and forth in large numbers, all the time.
A book has been written on the history of the boats and ferries that have operated on that route over the centuries: IRISH PASSENGER STEAMSHIP SERVICES VOL 1 North of Ireland DB McNeil 1969.
According to this book, up until 1830 was the era of the individual shipowner. Competition was fierce and some went out of business within months. In the 1820s few passengers booked. Most turned up at Belfast docks and took the next departure. Agents would tout for business and on at least one occasion an agent offered free food and free passage.
1830 saw Burns Line start to dominate the market. In 1882 they acquired the Ardrossan route and gradually took over most of the main services between Northern Ireland and the Clyde.