A brief explanation why Clan MacNeacail (MacNicol / Nicolson) and Clan Nicolson (lowland) are recognized as two separate clans.
I am the former President of Clan MacNicol in the USA, where I founded the Society about 14 years ago. I'm originally from Edinburgh, went to the Royal High School and the University, and then emigrated to the Boston area in the USA.
To explain why there are two clans: I've always been interested in genealogy, and in the early 80's got in touch with the fellow who is the Clan Chief in Australia, Ian Nicolson. His ancestor had emigrated from the Isle of Skye about 1825, from Scorrybreck near Portree. The family had lived at Portree for many generations before that, and had entertained Bonnie Prince Charlie during his flight in 1746 - he was hidden in their cow shed! I and a lady in Australia decided to get the Clan societies up and going, but in the process of stirring things up a rival emerged - Lord Carnock, AKA Sir David Nicolson of that Ilk claimed the title of Chief of Clan Nicolson, by virtue of an ancestor who was given the title of John Nicolson of that Ilk and a Baronet of Lasswade in 1629. He was recognized as Chief of Clan Nicolson by the Lord Lyon in 1984.
We discussed the clear claim of the Nicolson's of Scorrybreck to have a Chief, and in 1988 the Lord Lyon decided that our man could be titled Iain MacNeacail of MacNeacail and Scorrybreac, Chief of the Highland Clan MacNeacail. This required Ian to change his surname officially to MacNeacail from Nicolson - one is the Gaelic form of the other. This resulted in there being the two clans, and hence your problem about distinguishing them.
To establish the foundation of Clan MacNeacail, we commissioned a book to be written which was published in 1999, entitled The Highland Clan MacNeacail (MacNicol), by W.D.H. Sellar and Alasdair Maclean. Copies can be obtained from our Clan Society. We have branch societies in Scotland, Canada, USA, Australia and New Zealand. We also own 130 acres of land that we bought in 1987 near Portree.