All data has been derived from the transcriptions in the IGI derived from the official 1911 transcription - thus it is a transcription of a transcription of an earlier transcription !
Based on various Church, Civil and, post 1821, Decennial Census data
The peak in the 1831 census matches a dip in the Maughold census and probably indicates a confusion over some boundary region. The sharp drop between 1861 and 1871 probably reflects boundary changes with the growing Ramsey.
The register starts from 1696 - it is possible that the dip in baptisms reflects the re-establishment of Ballure Ramsey Chapel in 1748 but more likely are due to poor record keeping (also possibly similar IGI batch information removal - see Andreas).
Several epidemic peaks can be noted.
R.A. Houston The Population History of Britain and Ireland 1500-1750, Macmillan Education Studies in Economic and Social History (ISBN 0-333-56564-9) 1992