You haven’t made clear which record sets you are talking about.
In some areas Ancestry has advantageous access to original register images, and in others Findmypast has. FindMyPast’s Devon Marriages set is just one example of the latter.
I just mean marriage information in general. Did every wedding from 1837 to today have the form where it looks like this?:
https://www.irish-genealogy-toolkit.com/image-files/sophiadoolittlemarriagecert.jpg
You have given an Irish example. I know very little about Irish research.
In England and Wales there was a common form entry from 1 July 1837. The full details can in every case be acquired by way of civil certificate, to be ordered using index details from FreeBMD, GRO and the like (which are also available on commercial websites). The index itself just gives very basic information.
If the marriage was in a register office then the above is the only way to get the full details.
If it was in a C of E parish then you might be lucky and find that one of the commercial sites has paid ££££ in order to have the right to place digital images of their original parish registers online. Different companies have acquired commercial rights to different image sets. It is a competitive market between them.
If it was in a non C of E parish (Catholic/nonconformist) then there is less likelihood that the original registers have been imaged (the Catholic Church, for example, has not permitted this in many cases) but again there is a commercial imperative for the paid sites to acquire access to whatever they can get, and coverage is improving all the time.