1851 = census year
Patrick = Parish
District 3 = the third district of the Parish
Schedule 49 = the number of the form or schedule that listed the household. The schedules of a district were numbered and entered into the census books in number order.
Rheaby = address

IGI = International Genealogica Index
go to www.familysearch.org
to use the index. In this case the date of marriage and baptism of children can be found.