As a McCormick, I believe that I am able to join the clan. Although I am from Irish Ancestry.
McCormicks may indeed join Clan Maclaine of Lochbuie - in fact the MacCormicks have been recognised as a senior sept of the Lochbuies since 1540. In fact the MacCormicks were directly responsible in assisting the 6th Lochbuie chief to recapture Moy Castle.
The MacCormick roots are in Ireland - as were the Maclaines/Macleans in the early 1200s - but since 1540, they have been considered a Scottish clan, and cadets of Lochbuie. As you probably know, the MacCormicks are entitled to wear Maclaine of Lochbuie tartan, and use the Battle-axe as their crest. In the early 1900s, a MacCormick specific tartan originated, and pipe music to them was composed.