Then there’s the tunes that are second in a set and you can’t play as a standalone tune. You have to play the first one before it registers.
There’s the tunes you know until you get to the end of the B part, then you don’t.
Yes. 
Or even the A part.
As a child I found that music helped me memorise words (songs committed to memory easier than poems), and as an adult that words helped with music (songs memorised easier than tunes).
Working to expand my repertoire, it seemed an advantage that many ITM tunes used different combinations of the same few dozen phrases. Unfortunately I’ve allowed this “advantage” to develop into a problem by failing to reinforce the mental links between A and B parts of a tune. There are no local sessions, so I rarely play with others and I am now quite prone to start a tune but finish it with a B part that, though it flows on naturally, comes from a different tune entirely.
Without ABC or staff-notation as an aide memoire, I now string together mixed medleys that sound “right” to me, but owe more to an MP3 “shuffle” button than to the tunes the various parts originate from. I’d be in trouble in a session 
so it looks like ad nauseam repetition of a single tune will be the eventual answer (when I get round to it!). There are no local Trad Police, though so until then, ignorance is bliss and my favourite set is “What diddly, diddly dee tune, and was that X?". 
And that’s probably what a lot of sets sound like to the casual listener anyway, so hey! Folks like us are just taking the direct route. 
Yup! i still can’t wrap my head around The Green Hills of Tyrol and… that other tune that sounds just like The Green Hills of Tyrol except a couple things are different. 
Every time I think I hear Green Hills I get excited 'cause I like the tune… and then half the time it’s the other one, which I don’t like so well… Why not just play Green Hills, then? Why???
I remember when I first started playing at sessions I would faithfully ask what a tune’s name was and write it down somewhere. Whenever someone, especially one of my heroes, would say “I don’t know what that’s called” I’d be amazed. I mean, HOW could they sound so BRILLIANT and NOT KNOW WTH THEY WERE PLAYING???
Alas, I still don’t have the wonderful part but I have proudly achieved the “Not Knowing WTH This Tune I’m Playing Is” pinnacle. I hope that counts as progress, but I’m afraid all it really means is that my head is full. Like Celticexile, I blame it on the iPod and “shuffle.” Now they’re all just called “this one … this one … and this one.”
A local gal, brilliant player, is so famous for not knowing tune names that we practically call the press when she has one. At a party one fellow asked her what a tune’s name was, and she snapped, “How the f@&% should I know?”
I’m forgetting tune names. I hope it’s just the sign of an overflowing inner library, and not my greying head.
The thing is these days I mainly get my tunes from old session recordings or Comhaltas clips online, and the tune names are rarely mentioned. I need to make my own research, using Tunepal, to find what the tune is and who composed it. I don’t care much about titles, but I care a lot about who composed the tune. I love it when I find out I’m playing a Ed Reavy, Coleman, Paddy Fahy, Junior Crehan, Paddy O’Brien etc tune. It’s also very interesting to find the history of a tune, and in what key it was originally composed (too many tunes have been converted to the key of Am, Em, G, etc to make fluters and pipers happy!).
Tune names are great for communicating with fellow sessioners though, sometimes when you want to discuss what to play together.
Nah, at the end of the day I’m a fan of the lingua franca, which is to say the beginning notes: “What shall we play?” “How about [twiddly twiddly] and [twiddly twiddly]?” “Sounds good. What else?” “I’m stuck.” “How about [twiddly twiddly], then?” “Brilliant. Hup…”
For me, tune names have almost grown into just an attached curiosity, because one tune can have so many of them. When someone asked if I knew Donnybrook Fair, at the time that name was unknown to me, so I could only reply, “I dunno; play it for me.” And lo and behold, I knew it, but not by that name. Ah, well; I suppose it all works out in the end.
Yep, it works most of the time, but there are occasions where you want to call the name of the next tune in your set in situations where you want a clean switch to the new tune. Most of the time I’ll only call the key for the guitar dude, but yelling the next tune name has been useful in the past
I really wish people wouldn’t call the names of tunes in a set, because it always sounds like “Bleurrrrghhhhhh!” And I’m like, “What?” And they’ll say again, “Bleeurrrghhhhh!” And I’ll say “Sorry, what?” Then the tune will change and the change will be even more of a trainwreck because I’ve been shouting “what?” and trying to figure out what the set leader was saying, rather than thinking about playing.
Me, I don’t say anything. I expect people to be psychic and know when I’m going to change tunes. I also rarely know the tune I’m going to change into until the last time through the B part, the third time through. Or not, in which case the tune gets played a fourth or fifth time. I’m wonderfully unpredictable in a session; no one, least of all me, knows what I’m going to do.
This is what makes a session so cool! I heard of sessions where people would discuss what they’re going to be playing in a set every time. 
I’m guessing it’s to accommodate those sheet music players. 