I think there was an error in the way that the marriage has been entered. As illustrated by the rest of the entries on the page of register, witnesses' names were not included here.
Normally when witnesses are listed, the entry starts with the word (witnesses) abreviated or in full and then the name of each witness and where they came from.....
I see what you mean - and something else has now occurred to me. Compared to the other entries, this one looks rather faint. Could it be that the clerk started to write the entry, realised it was a mistake, and then erased it as best he could?
One possible reason that occurred to me was that he was recording a baptism, and realised he'd started to enter it in the wrong book. I was going to suggest looking to see if there might be a baptism that would fit with this explanation, and then noticed that Debra had already suggested one:
The surname is looks like MACKAE not MACRAE. There are are other events in Bracadale recorded as MACKAY so probably unlikely that their surname would be spelt differently. Perhaps this one is also MACRAE.....
Possibly a son baptised also in 1818.
MACKAE, ANGUS
ALEXANDER MAC KAE MARGT. MAC NIEL FR 25 (FR25)
17/09/1818
109 10/17
Bracadale
Debra 