I have added to those marriages my reading of the official transcript of Parish Reg for 1718 to 1800 as the IGI appears to be something of a mess.

My guess is that there are some 120 or so missing marriages in this period - based on other work my guess is that about 20% of these marriages are for families residing in adjacent parishes - however having two adjacent parishes (Onchan + Lonan) which did not give full names of parents in baptismal register complicates matters.