Max,
There are no records of travel between Ireland & Scotland. It was a short domestic journey, and no passenger records have ever been kept for that type of travel.
There were regular sailings from Belfast and Larne to Scotland (often 2 or 3 a day) plus at least 2 a week from Ballycastle to Glasgow. Folk went back and forth all the time. The fares were low and competitive. Folk were able to come home to Ireland regularly for the Glasgow Fair and other holidays, or to help with the Irish harvest.
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, 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. (The main business being cargo with passengers being just a potential source of top up revenue).
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.
Evidently David isn’t in the 1861 Scottish census but obviously was in Scotland by 1869 to marry. Most people went to Scotland for work, and that’d be around age 15 or 16. So I would guess David left around 1863 – 65, but that’s just a guess. I doubt you will narrow it down anything more than that.
There was a railway station at Killagan (43 miles from Belfast) which opened in 1855 at which time it was named Bellaghy. Name changed to Killagan in the 1870s. Photo of the station attached. The station closed in 1973 but the line is still open. A train from there to Belfast probably took around 90 minutes. My guess is David caught a train from Bellaghy/Killagan down to Belfast and then an overnight sailing to Greenock or Glasgow (Broomilaw Quay).