Can anyone please help?

I'm trying trace where my ancestors may have disappeared to. I've managed to get quite a lot of info through Census and Birth data and even spent a lot of time on the Scottish People computers at the local library but I'm hitting my head against a brick wall now.
Elisabeth McDonald, B-About 1850
Robert McDonald, B-About 1852
Agness McDonald, B-About 1855
Andrew McDonald, B-17th Dec 1858
They are son's and daughter's of a Hugh McDonald
(Born somewhere in Ireland 1819 and dies in Kilbirnie in 1861)
and...
A Jane Lang
(Born in Kilbirnie, Ayrshire 18th June 1824 and dies in 1899)
Elisabeth McDonaldI have Elisabeth living in Kilbirnie in the 1851, 1861 and 1881 Census and then nothing
At this point she would be about 31 yo
Robert McDonaldI have Robert living in Kilbirnie in the 1861 Census
and then possibly Kirkcaldy, Fife in 1871 and then possibly Glasgow in 1881 although nothing concrete yet to prove this. There is also the possibility he may have married a Mary Ann cochrane which I'm now looking into, but again no hard evidence. He would be about 29 yo in 1881
Agness McDonaldIn 1883 I have Agness marrying a James Logan in Kilbirnie on the 27th Nov but have nothing after that for either of them.
Andrew McDonaldThis is the person I am mostly interested in. I have my 3rd Great Uncle in Kilbirnie in the 1861 and 1871 Census and then i believe I have him in the 1881 Census in North Berwick, East Lothian as a teacher. (Common in my family) After this and endless searches I can't find anything but my thoughts are that he may have travelled overseas... He would be 23 yo in 1881
Apparently one of my great(x4) grandfathers was a poet but I have also yet to unlock this mystery although some of my relatives who are born in the early-mid 1800's become quite well established (locally) in that field. The furthest I can get back is a Hugh McDonald in Ireland (probably born around 1800 who marries an Elisabeth Elliott. I have absolutely no information on any of them so don't know if their son Hugh travelled alone to Scotland or with family.
Any help whatsoever on any of the above would be of great benefit to me.
Thanks in advance for any advice, help or input.
Alec