The editors at the
WAT (Afrikaans equivalent of the Oxford Dictionary) are of the opinion that it's a phonetic version of
Ruadhan - itself a diminutive of
Ruadh =
Roy, and with the -dh- silent. (Ruan sounds pretty much like ROOahn, with a long a.)
And this may well be so; and of course then the meaning would simply be "red" (or, would be Simply
Red); however, most names used in English tend not to cross over into Afrikaans and we don't have a strong Irish presence here at all.
So, my view is that while Ruan and
Ruadhan sound pretty much identical, the reason for Ruan's popularity (and it is widely used, in various spelling variations though never the Gaelic one) could well be its resemblance to a merger between Ru(dolf) and (Joh)an.
The Dutch are inclined to string names together and use the whole string:
Geert Jan and
Jan Harm for instance. An English analogy would be
Mary Ann. But the Afrikaans seem to prefer merging names to create a new entity: as the French and the English have with
Marianne. Or
Charlize; or a woman I used to know who went by Marelna, though her names were
Maria Elisabeth Helena. Given a small population with a restricted name stock and a strong tradition of recycling the names of grandparents, mergers make sense, and would account for the warm welcome given to Ruan and the total failure to adopt, say,
Duncan, which doesn't fit into the traditional naming pattern at all, although as Celtic-fringe names go, it's better known that Ru(adh)an.