I use a couple of classically trained keyboard players in the barn dances I do - good players both - but it took a while for them to relax into the freedom that folk music gives you. I expect each time through the tune it should sound a bit different- but I still like to be able to hear the tune. Thing is - if you do a song just as you have heard it - you may as well just as well put the CD on.
BUT - if you are playing in a pub for money- non folkey audience mostly- they will want to hear whiskey in the jar etc as they know it. Dont put in variations except perhaps a few twidley bits with an instrument. Singing as you heard it also gives you confidence. Also in some sessions however you sing it the others may well sing the choras as they know it. We use the older version of new york girls - you sure do like the money - but are often drowned out by people singing - sure do like to polka.

If you mention the area you live in on this site locals will tell you about the most welcoming sessions. Some sessions are very serious - for near proffessional singers. Most are more relaxed - and welcome newcomers. Some sessions are run ( organised ) by someone so look out to catch their eye. Others ( fewer ) - you just jump in. Some of the best sessions are run by morris sides after the dancing - but some morris sides just dance and then go home. If you only have one song it is OK to say so.

Good luck with your singing.
FloraG.