Civil registration in the IOM didn't start till 1878 (someone will jump in and correct me if that isn't quite right). If you send off for a certificate for events before then you'll get a copy of the parish register entry, which in the case of births and marriages won't necessarily give you all the information you'd expect from a UK cert of the same date, or from a Manx cert after 1878.

If you have a Mormon family history place near you, you can hire filmed copies of the registers and see everything for yourself at a fraction of the cost.