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
Castletown is wholly within Malew.
The register for Kirk Malew starts from 1648 but that for the pervious 5 years (i.e. 1643 onwards) was recently found. Castletown Chapel records a few baptisms prior to its rebuilding in 1826 after which it appears to be the church of choice for those living in Castletown. St Mark's was built in 1774; the erratic nature of the baptisms is an artifact of the IGI removing batch numbers (still under investigation - see Andreas for details).
The peak in 1821 seems to include several non-Manx names, possibly soldiers from the garrison ?
Several epidemic peaks can be noted - the 1741 in particular, the 1832 Cholera peak is also noticeable, whereas the 1837 typhus peak seen elsewhere is much smaller.
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