here are some links to a more scholarly examination of the Irish Famine than what is currently occupying this thread.

http://xroads.virginia.edu/~hyper/sadlier/irish/rwhyte.htm

http://xroads.virginia.edu/~hyper/sadlier/irish/Famine.htm

"Census figures put the population of Ireland in 1841 at 8.2 million. The Census for 1851 tallied 6.9 million. That marks a drop in population of 1.3 million and that number is not solely down to the decrease due to the famine it also includes those who died of natural causes unconnected to the famine - True?"

oh... and comparing the census data from 1841 to 1851 is as meaningful as the answer 42 in the Hitchhiker's Guide.

Two data points without context is absurd... and no population biologist would ignore reproductive rates along with migration (both in and out) and mortality when positing their conclusions. Because your statement is only valid if in that entire decade no children were born or died... not the way I would bet.

Populations are not static... that is why you need to apply statistical tools with well defined assumptions... not "simple arithmatic". That only is valid to support the statement that the change in population is from this to that... and useless to expalin the "why" for the change.   

The 1841 figure includes all those born and still alive at the time of the census taking. And children born subsequently are only tallied if still alive and living in the census area. You need to know the reproductive rate of the studied population to infer approximate numbers... or hope that the birth/baptismal and death/burial records of that period exist and are reasonably accurate... to support a statement regarding just how many individuals perished or migrated.