This has already been pointed out in a comment on the wikipedia article, but it does not seem to have been mentioned here yet, so I will.

In short, the story as presented here does not add up. Looking at the 1901 Census of Ireland, there is a Mary MacNamara living in Spancilhill who by her given age must have been born in 1865. There is also a John Considine (well, four of them, actually, but this seems to be the only real candidate), son of Kate Considine who by his given age must have been born in 1875 or 1876. Assuming these are the right people, which seems likely, the claim that Michael Considine was born in 1850 and died in 1873 makes no sense. Mary would have been only 5 when Michael left (when he was supposedly 20) and John would not even have been born when Michael died.

Now, I noticed that when Mr. McGrath originally posted the full lyrics of the song, Michael Considine's year of birth was given as 1859, although this was later corrected to 1850. If 1859 should turn out to be the real year of birth, however, the other information would seem to add up. Assuming Michael still left for the US at age 20 and died at age 23, Mary would only have been 5 years younger, which gives an age of about 15 when Michael left and John would in fact be six years old when he received the poem, rather than being not yet even conceived.

Does this make any sense, or is there some other explanation I've missed?