Hi Colin
If I were you I would be looking at all of the revision books and having a look at any changes over the years. Revisions were done approx every 10 yrs but this did vary. Any changes of tax or tenancy were noted at this time.
The notation on this document is quite interesting it reads something like: James Millan (?) claims to be

instead of M?

? or

?? John Ferguson for Rob Ferguson for 1862 John Ferguson dec.
So really you need to find out more.
At the beginning James McClenn jun is crossed out and John Ferguson written above showing a xfer of tenancy.
I would also be looking at the estate of The Hon. Somerset Maxwell. For example:
http://sources.nli.ie/Record/MS_UR_077926All the GV information, revision book info + dates, any parish or Civil reg information should give you a better idea of family connections.
Good luck
CD
Note: There is a James Mullen listed on the GV on Plot 3 in Tullyboy - so perhaps this is the man disputing the claim of Ferguson?