Wm Ellison is possibly the Wm Ellison who m Ann Cannell in German in 1795 (he signed reg so could write) - his fam is in German (?Peel) thus suspect John Shimmin came from German; I havn't looked at Patrick marriages in detail for this period so not sure if 2nd witt was parish clerk or a friend/reln.
As a sidenote it is always worth checking witnesses as those who sign with a X are almost certainly connected to one of the parties.