I have many many reasons for learning new songs, but getting requests for them is always low on the list. First of all, there are too many to learn them all. Second, do I want to spend all that energy and thought on a song I don't like and therefor, probably won't sing well? When someone I know suggests a song repeatedly, I will often look into it and see if it is one I would enjoy singing.   

However, in an effort to say something useful on this thread. Have you thought of your songs in categories? If someone asks for a song you don't know, it might be possible to sing them one from the same era, similar background, similar message, originating in the same County, or something else that makes the requester feel that they have been attended to, without compromising your performance by singing songs you don't know well or don't like. Here are some examples... but I don't know the standard's well enough to do scads of these. Have fun, Claire

Request             Substitute                Reason
Fields of Athenry    Green Field of Amerikay    another field
Danny Boy            You're not Irish          song about Danny Boy
Molly Malone         Irish Molly O             Another Molly