There are four McClelland family trees for Co Down all in Volume 1 with starting page numbers as follows in 1p8 Banbridge, 1p20 Lisnaree, 1p22 Baron, 1p24 Alexander, This volume is in Blackwood Family history manuscript trees in the LinenHall library, Belfast. Its easiest to visit, however no harm enquiring.
I suggest you ask them for photocopying estimates if they think your knowledge relevant and be prepared to pay the extra for post and packing. However if the material is really relevant when you get it, I suggest a further donation to this unique library that is always needing extra funds. Linen Hall Library,17 Donegall Square North, Belfast BT15GB, Northern Ireland, Telephone: +44 (0)28 9032 1707, Fax: +44 (0)28 9043 8586, Email:info[at]linenhall.com
Have you also looked at Ros Davies website ie freepages.genealogy.rootsweb.com/~rosdavies/
Have you also looked at
www.familysearch.comThere will be many more sources.
Also try the Presbyterian Historical Society by letter, as one of their past librarians was an Aiken McClelland who was very interested in genealogy.
Hope this starts you. Jim