99.99% of records will be in the Manx National Heritage Library (in Manx Museum Douglas) open 10am to 5pm Mon to Sat.
The only exception is the original Parish register for Patrick which as of last week still hadn't been deposited in the library - it should have been there a year or so ago by law - however there is a transcription on film (which like all the other 1911 official transcripts are available on LDS microfim).

You may also like to call in IoM FHS library in Peel open 2pm to 5pm Tues.,Weds & Sat (above Shoprite in Michael St). With a Peel connection also try the Leece Museum on the Quay - not sure of opening hours out of season but they have a web presence. There is of course many other things to see in the sunset city - check with MNH re opening hours of Castle if you are out of season.