Another problem I've experienced.

I play an autoharp custom built by a guy named Brinker in PA (sort of a Fladmark acolyte, but willing to do a custom celtic drone tuning and bars). The cheap way involves getting a Radio Shack lavaliere mic ($30) and string it over your left shoulder so that it is positioned behind the 'harp. [Bryan Bowers uses a similar set-up with a Shure choral mike].

If you need to switch out from the 'harp to something else, get a Morley A-B-AB switch ('bout$40) and run the harp mic to the A input and your other mic into the B side of the Morley (it's a 1/4" TS connection so pick up a 15 foot Female XLR to TS Male, to go from the back of the mic into the Morley). Then run the Morley to a good DI box to go back along XLR lines to the Mixer (I recommend the Behringer DI 100 - $65). You will need a short 1/4" to 1/4" cable to the DI, and an XLR line back to the mixer. DI's convert 1/4" Tip Sleeve input to 3 line XLR low impedance lines.

If you are within 20 feet of the mixer you will not need the DI (Direct Inject) box and just use a 20 foot 1/4 to 1/4 TS cable to the mixer. The Morley will allow you to use both the 'harp mic and the other mic (AB) or either of the mics separate (A or B). BUT, because you will not be able to attenuate either the mic or the lavalier (or pick-up - see below) UNLESS they are in the A or B mode, if you have the extra lines to the mixer always use them.

I know this sounds confusing, but I don't have a lot of time here, so have a sound person or music store sales person explain and demonstrate the above.

NOTE: the RS lavalier mic will feed-back if the 'harp is not carefully taken off your shoulder. The alternate that I use myself, is a Dean Markley Acoustic pick-up ($50) which can be used on most acoustic instuments, but sounds very nice on an autoharp. It's a round wooden covered device that sticks to the back of your instrument with a special putty that is removeable. Experiment with the harp's sweet spot (which, for mine, is four inches up the back from the bottom of the harp, and four inches in from the bass side). Then send it to a DI and back to the mixer.

Finally, if you only have one mic to work with all your instruments ( like an autoharp, banjo, bodhran, and guitar - my personal set up) and you do not have a sound person to change the gain with each instrument, do the following:

One Shure 57 mic on a boom set to the front (for the 'harp set high aimed 45% down to the sound hole - other instruments, aim above the fingers - experiment, etc.) Get the 15 foot XLR-F to TS-M cord mentioned above to a volumn pedal (Morley makes one, as do others). Use the short TS to TS to get to a DI and from there to the mixer on an XLR. Follow the pedal directions to set the volumn pedal to the minimum for the banjo or bodhran and the maximum for the autoharp. Then play with it 'till you get the settings right. The EQ will suffer, but chop the bottom off the bodhran setting or the bass proximity of the guitar, and pull enough highs to get rid of the string strike and clatter of the harps chord bars, and you should be OK.

Again, if you have problems understanding the above, hie thee to a sound man... Good Luck - gotta go...