Well I know some acts which are purely instrumental, smallpiper spring to mind, and what makes it acceptable is good stage presence and plenty of interesting in-between patter.

As for learning to sing, there is rafts of info on the web - and it needs as much commitment to sing well as to play an instrument well.

Important things for instrumentalists are (IMO)..
1) learn to sing *without* your instrument
2) singing, at least in folk music, is never secondary, so if you choose to do it, then on those numbers you should think of your instruments as background to the song.

Jim