It sounds as if at least some of the occasions you mention, Jimmy, were pubs where the owner has paid a group to provide Irish Atmosphere. The agenda of the group is therefore to "sound irish" and sing some "Irish songs" (you know the type - Dirty Old Town - written about Salford, Green Fields of France - by a Scot living in Australia).
In many such situations the assumption is that the music or song is there just as a background to whatever conversation and drinking there is going on. Nobody is actually expected to listen closely to what is being sung, so why should it matter if the odd verse is missed. Who really cares?
I generally would prefer to be in a situation where the band or singer is expecting people to actually listen and therefore treats the material with a bit more respect.
That said, sometimes I find it more annoying when somebody insists on adding verses which do not belong to a song (here is an extra verse I have written because I am a self important prig) as they usually spoil the original.
Quack!
GtD.