Re. the 1881 census, FamilySearch has his occupation as "mason":

William COWELL Head M Male 44 Douglas, Isle of Man, England Mason
Jane COWELL Wife M Female 29 Douglas, Isle of Man, England
Emma COWELL Daur Female 14 Douglas, Isle of Man, England Scholar
Ada COWELL Daur Female 8 Douglas, Isle of Man, England Scholar
Christian COWELL Daur Female 4 Douglas, Isle of Man, England
Sarah COWELL Daur Female 1 Douglas, Isle of Man, England
Source Information:
Dwelling No 1 Thornhill
Census Place Onchan, Isle of Man, Isle of Man
Family History Library Film 1342346
Public Records Office Reference RG11
Piece / Folio 5602 / 105
Page Number 3

With a 14 yr old daughter that puts their marriage a bit earlier than the 1870s, except with Jane aged only 29, perhaps his second marriage?

I'd look at this one on the parish register [IGI]: WILLIAM COWELL Spouse: JANE CROWELL Marriage: 20 JUN 1874 Braddan, which to me looks like a mistranscription.
LDS film no. 0106190.

It looks as though the IGI bapts of the older children from the parish register transcripts have been removed and replaced by members' entries who have picked up the "Jane" from censuses.

Sue