I find a certain amount of 'inference' is involved, particularly over the periods of mass migration, like during the potato famine (~1845 to 1852). Frequently there is no embarkation record to match up with the records at this end. Many births and deaths also occurred in the Irish 'poor houses', and went unregistered.
Many names of that era were also very similar … sometimes the firstborn 'Jr', was given the same name as the 'Senior' …. and mis-spelt surnames and unclear handwriting is very frequent in 'official' records. This happened at both ends. Church fires resulted in big gaps in the birth records at the Irish end, also.
Sometimes there's a big temptation to associate the 'nearest' person identified from a record, with the person one is searching for.
Much identification depends on the assumption that few people moved locations back in those times, also.
Correlation of a birth and death record, and its applicability to the person being searched for, frequently involves a certain amount of guesswork.
Cheers
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