IOM civil registration of births and deaths started in 1878, and marriages in 1884. Adoptions from 1928. As you can see from Brian Lawson's registration indexes for births and marriages there were a few earlier ones recorded, but registration was not compulsory before the above dates.
www.gov.im/registries/general/civilregistry/
www.iomfhs.im/lawsons/LawsonBMD/Main_Site_index.html

If you can't find a birth listed within BL's dates put it on the message board, because extra ones are on Ian Radcliffe's website but the link seems to be broken (I'm sure he will fix it shortly). lawsonsiom.freevar.com/

Sue