I checked the Tithe listings for Co. Sligo and there were seven McKeen records but no Samuel or William and no one in Largan or Dailly.
The 1858 Griffiths Valuation head of household listing for Co. Sligo showed five Mackey records but no Samuel or William.
On the subscription site Roots Ireland there was a Church of Ireland baptismal record in 1856 for a Samuel McKee but his father was Robert. Many of the Co. Sligo C of I parishes ( I presume your family was C of I) have records starting late e.g. 1870s.
There are two Largan townlands in Co. Sligo but I was not able to find a townland spelled Dailly. Perhaps it was a local place name.
Have you tested your DNA?