That is a good point! I hope some day to be able to actively contribute rather than just play along.
Do it. Do it now. You said it - confidence. That’s the name of the game. I would stop when the mail person would drive up in their truck to the mailbox. Then one time I was playing when I thought nobody was paying attention and a guy walked down the street and I stopped. He said “Don’t stop on my account. I was enjoying it.”
Since then I have learned that people enjoy you putting something out there, warts and all. They respect that, and enjoy it so long as it’s not horrible. They remember the good bits and don’t even realize you’ve goofed up most of the time. The trick is not to stop when we mess up. I used to stop and go again when I was practicing but I found that made a habbit that was no good when playing in a session. Just keep the tune running in your head as if there are others there and practice picking it up again when you can.
Works for me, YMMV. But you won’t get over playing in public without playing in public. So play in public!!
I sometimes will just skip any notes near that part of the scale because I can’t remember which C to use. I totally relate.
But have you ever noticed there are some tunes where the Cs could go either way: C or C#? It’s almost like neither C sounds quite right and neither C sounds quite wrong.
Thanks for all the help guys. I knew I could count on c&f. I had a beer yesterday (dutch courage and all) and I played outside and a bit today just on my boat! There were 5 people out and about the marina. At first I played slowly and repeated a tune until I relaxed. As I relaxed and began to focus on the music I changed to a session like style, playing a tune 3 times through or just twice, depending on the tune. I managed to hold it together when I messed up a couple of times. 
A fellow boater asked me later what that was I was playing and after I told him a tinwhistle/pennywhistle he said and I quote, “well that was just beautiful” and it reminded him he was Irish (descent)!
sbfluter, thanks for your post I am glad I am not the only that confuses the Cs!
Thanks for everyones help again, I now know what to aim for regarding session play and after reading some old posts about performance anxiety I have my work cut out. Lucky I love this “work”. 
Yes Cs are funny things in Irish music, perhaps especially on the pipes.
On the pipes there are certain note-sequences where pipers will usually play C#, others where they will usually play C naturals.
So, the pitch of C doesn’t have much to do with a concept of what key a particular tune is in, but rather the shape of the melody.
The sequence back D, C, A is usually played with a staccato (or at least detached) C sharp.
When C is approached from below, especially from B, the C is usually natural.
Here’s a bit of a jig that shows this:
dC#A A2Cnat/ dC#A GEB/ dC#A A2D/ F#AD GED/
So you see on these pipe tunes any attempt to decide which “key” the tune is in is pointless- a guitarist I know would call it “D-ish”.
The D mixolodian mode (listen to Banish Misfortune) switches between Cnat and C#.
Chord players can tell a mixolodian mode by the frequent changes to the base chord to one that is a whole step down.
A mix for example will have changes from A to G chords. You use the standard D fingering and the G note is played natural, even though it is D-ish.
There is quite a bit of information on the different modes common in Scottish/Irish trad. Just Google a little.
My experiences are in total agreement with Rob’s. In Ireland or among off-the-AirBus Irish or first generation musicians, it seems pretty normal to play a tune a MINIMUM of three times. Shorter tunes like polkas or two-part single reels can get played 6 or 8 times at a go.
I like Rob’s description, too, especially the “conversation”, and not just in Ireland. The 2x/3x convention is a good starting point, but not a rule.
Sometimes different instruments are dropping in and out, or people are actually switching instruments, and each rep becomes an exploration of a different texture. Sometimes people are acquiring the tune, and more reps give more learning time. Sometimes players are exploring variations. Sometimes several players will lock into a groove, or an accompanist and lead players, and each rep brings out a different personality of the tune.
It can be frustrating when some players want to switch tunes mechanically after 3 reps, while others are just hitting their stride. Good session communication, both verbal and nonverbal, is the key to sorting this out.
I wonder how much the conventions have been influenced by recordings, and the need to squeeze sets into nice 3-5 minute chunks. A reel or jig typically takes around 30 or 40 seconds per repetition. So you can do the math to figure out how reps would be limited in a set of 2 or 3 tunes. Outside the core ITM areas, players who source their tunes mainly from recordings might feel more artificially constrained by what they’re simply used to hearing.
So my solo sessions go a bit like this now:-
3 repeats of tune most sessions experimenting with home made sets.
Then other days I get into a stride after 3 repeats and want to do more of a tune. So I say to myself, “Self, lets do it the Irish way and forget this generic bog standard routine!” and we do.
Good on ya Boatgirl!!