HA
Yes, they were SUPPOSED to register but a LOT didn't.
With rural families, if they married in Winter time and were far from the registry office then it usually wasn't done but there were lots of other reasons too I'm sure.
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familysearch is the best free site for looking up records.
Most others require a fee or subscription.
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It's unusual though ~ both census have Martin as a farmer and normally you would expect to see either his or his wife's family in the area yet NEITHER surname seem to feature in Portumna.
You did mention that family have said that this family were originally from Ahascragh which by today's standards is not that far away, but for back then it would have been a big move.
Still unusual for a man to farm so far away from his family roots ~ I'm wondering maybe was he left the farm in Portumna by a family member ?
Looking at Ahascragh on the census though, there are one or two families with these surnames in the area ~ not what I would call local surnames though.
Looking at the GV for Ahascragh there are a few references to the surname Carroll BUT interesting there is NO reference to the surname Fitzgerald.
It all might explain why you are finding it so hard to find the marriage
Tara