Johnston's
Place Names of Scotland suggests as the origin of Ardersier either
árd ros ear meaning 'high east point' or
árd an saoir meaning ' point of the carpenter' but in so going he confuses
árd, which specifically means high or tall, with
ros, which is a promontory or peninsula. (Also, I think that in order to comply with the rules of Gaelic orthography it would have to be written
árd an t-saoir which would anglicise as Ardentyre, by analogy with the surname
Mac an t-saoir which anglicises as Macintyre.)
Not surprisingly, Johnston is now regarded as unreliable.
I don't know the origin of the name 'Cromal', but I'd be surprised if Cromal Mount were the
árd in Ardersier because it's not a notably high or prominent feature - it seems to be built on, or part of, the raised beach that runs roughly north-south above and to the east of the village. If it were a notable feature of the local topography there would be a photograph here
http://www.geograph.org.uk/search.php?i=123595917Canmore
https://canmore.org.uk/site/14316/cromal-mount-ardersier suggests that it may have been a mediaeval motte, and that 'Cromwell's Mount' is a popular corruption of the original name Cromal. Obviously, if it's mediaeval, it pre-dates Oliver Cromwell (1599-1658) by some considerable time.