Don't believe everything you see on ancestry - you are accepting the work of some other person, do they give any documentation that supports the manx backround - do they give an email address for you to contact them ?
a quick look at the IGI suggests born 07 APR 1825 Kingsteignton, Devon, England a mere 200 miles and much water away from IoM - try going to IGI select british isles then trace Thomas Mann bap/birth 1825 exact year