When you look at the IGI entry, check under "Messages" lower down the screen to see whether it is an extracted record (extracted from the parish register). If it is a Church member's entry it is unreliable.

Below "Messages" is "Source Information". See "Source Call No." which gives the LDS film number for the entry you are looking at. For example, the marriage you mentioned between Thomas Cottier and Mary Lawson 22 JUN 1833 Braddan has a list of films for Braddan, including Date: 1817 - 1844 [LSD film no. 0106710].

If you click on this film number it will tell you exactly what is on that film, under film notes.

However an 1833 marriage is unlikely to give you many further details about the couple, because occupations and fathers were not usually given until the late 1840s. Witnesses will be recorded, and whether anyone was underage [under 21] or from a different parish.
Baptisms sometimes have the residence recorded - and they are all worth looking at, rather than just an index.

Films can be ordered to view through any Family History Centre - see under Church of Jesus Christ of Latter Day Saints in the phone book.

Sue