In most of the sessions I go to in Dublin we start off with tunes, maybe for 30/40 minutes, then have songs for about an hour or so, then more tunes--this gives those people who remained quiet during the singing to have a chat--then more songs. This seems to keep most people happy (though there is sometimes heated discussion about presence, or lack of, of guitars, bodhráns, etc., but that's normal in a lively session!)