Hi Roger,
That possibility occured to me and I did contact Father Sean Henry - current parish priest for Dunshaughlin (which also covers Culmullen) - to ask him by email if he thought that might have occured.
He responded by saying he did not know but I was welcome to visit and view the entries in the parish register, provided I wore gloves when doing so.
I am not really sure when I might be able to do this as I live in Watford, England.
It might be worth examining the original parish register but I think the original entries were made in pencil.
At some later stage, I think the entries were then inked-over.
While inking-over has enhanced the visibility of many microfilmed copies, it may also be the case that entries have been changed by inking-over.
I suspect this may have happened in the case of the 1840 entry and a close examination of the entry might reveal what the original entry was.
Still, the fact that all names [child, father, mother, godfather, and godmother] in each of the 1839 and 1840 entries are exactly identical is very strange.
I have additionally scrutinised all the death entries microfilmed from the original parish entries and not seen anything about James Dowdall there.
Could there have been TWO James Dowdalls of different Dowdall families baptised within a year of each other?
It is also odd that James Dowdall ended up with a change of name to Dowdle after moving to England at some stage [not sure when].
I have seen a number of Dunshaughlin criminal petty court sessions involving his father William Dowdall.
Maybe James no longer wanted to be associated with the Dowdall family name and changed it to Dowdle when he migrated to England?
There are a whole lot of imponderables regarding James Dowdall, which does not make my research efforts at all easy!
Thank you for your interest,
John