As Frances said, the IGI that comes from extractions of parish records is relatively accurate.
The accuracy of records submitted by individuals varies widely.
The extractions have a CO (christening) and MO (marriage) prefix to the film #

There are some parish records that for various reasons were "missing".
The LDS library has several films of some of these missing marriages.
For example, marriages in Marown parish around 1800 aren't in the IGI but are on the "missing" films.

The IGI should always be used as a guide and needs to be verified by other records--wills, census data,etc.
Donna