boon = service - a boon day is when you had to provide a day of service (eg mending bridges etc) - eg see entries off www.manxnotebook.com/gazateer/ablands/index.htm.
I think the old Castletown deeds are those before the post 1702 arrangement for recording all transfers was in place - there are 'old deeds' for all parishes - the Castletown tag refers I think to the place where the court sat. (eg see www.manxnotebook.com/famhist/genealgy/deeds.htm) - some old deeds summarised at www,manxnotebook.com/cd_only/deeds/ln_old.htm