I think the thread asks what I wonder, what songs should should every whistler know how to play?
Someone is going to post a list of ITM songs. I’m going to tell you to learn Happy Birthday and songs kids knows. They’ll be your first fans.
At the risk of sounding like a broken record, I think clarification is in order. Do you mean tunes? Or are you specifically looking for song tunes? (The first would be everything that’s normally played on whistle within the Irish tradition; the second would be the tunes of songs, ie things with words and singing.)
Would that be a record of songs or of tunes? 
Go the The Session member’s page, and click on the Tunebook tab.
http://www.thesession.org/members/
The first few pages (20 per page) are very common session tunes that would be known - and even played 
- by virtually every session player.
That’s actually really really good. I literally LOL. Best grin all week.
Here are some of the popular ones at my old session:
REELS:
After the Battle of Aughrim
Blair Atholl
Brenda Stubberts
Castle Kelly
Congress
Cooley’s
Dick Gossip’s
Dinky Dorian’s
Father Kelly’s
Flowers of Edinburgh
Gravel Walks
Jackie Coleman’s
Julia Delaney
Maid Behind the Bar
Mason’s Apron
Mouth of the Tobique
Paddy on the Landfill
Pigeon on the Gate
Rakish Paddy
Ships Are Sailing
Silver Spear
St. Anne’s
Star of Munster
Swinging on the Gate
Tam Lin
Temperance
Wise Maid
Woodchopper
JIGS:
Banish Misfortune
Blackthorn Stick
Calliope House
Cliff of Moher
Collin’s
Connaughtman’s Rambles
Dusty Windowsills
Garret Barry’s
Kesh
Lark in the Morning
Merrily Kiss the Quaker’s Wife
Morrison’s
My Darling Asleep
O’Keefe’s Slide
Off She Goes
Out on the Ocean
Road to Lisdoonvarna
Sean Ryan’s (Castle Jig)
Swallowtail
Tar Road to Sligo
Tobin’s
Trip to Sligo
Willie Coleman’s
There are, of course, lots of other tunes that we play, but this should get you started.
MT is exactly right: you can use TheSession’s tunebooks to discover which tunes are the most commonly appearing in member’s tunebooks, which is probably the best international survey possible of the vague, unwritten, and ever-shifting ITM session repertoire.
When ranked in order of universality/commonness I discovered that of the first 100 tunes I knew all but a dozen, of the first 200 all but around thirty. So I started working on some of those!
Of course that’s a guide to the worldwide session repertoire as a whole. Fact is, each session is a law unto itself and will have its own collection of favourites. A particular session might even include a few tunes written by session members 
So the “real” answer to would to go to the local session you plan on attending, bring a recording device, and start learning their repertoire and their versions of the tunes (because it’s not just a matter of which tunes you play but also HOW you play them).
TWC list of Jigs seems to be full of easy ones and the list of Reels seems to be full of stinkers.
Or is that just me?
Anyway, wot, no polkas? No Slip Jigs? No Airs? The version of Merrily Kissed the Quakers Wife is a Slide, as I learned it. Still, the jigs will get you started. The reels might just finish you off.
I would think that lists of ‘most often played tunes’ would be heavy on easy ones as more people would know them.
Lots of nice reels there I think. Some I don’t know and cannot say.
On that list I like:
Castle Kelly
Congress
Cooley’s
Dick Gossip’s
Father Kelly’s
Maid Behind the Bar
Mason’s Apron
Pigeon on the Gate
Rakish Paddy
Ships Are Sailing
Silver Spear
St. Anne’s
Star of Munster
Swinging on the Gate
Wise Maid
That is a Tampa session for you I guess The Butterfly would be an appropriate Slip Jig.
Seems like the sessions I’ve attended over the last 30 years or so are usually around 70% reels, the rest normal double jigs.
Other dance idioms like polkas, mazurkas, single jigs, slip jigs, waltzes, etc all together make up around 5%. I’ve certainly been to many sessions where nothing but reels and double jigs were played.
Start an air and you’ll see faces that look kind of like this 
Though each session is different, and the repertoire of each session is of course the sum of which tunes the attendees want to play.
So some of the people I play with like polkas and we’ll play several. There’s a guy who likes Shetland tunes and we’ll play some of those when he’s there. I have Scottish smallpipes and some of the people have learned some Scottish pipe tunes from me and we’ll play those sometimes. There are all sorts of things that can happen, for example I was at one session where there were four or five people who had come down from Alaska (as I recall) who in addition to Irish music liked to play Bulgarian tunes and at various times in the Irish session they would launch into a Kopanitsa or Buchimish or whatever. (If you want to say “hey! but those aren’t Irish tunes!” note that in the lists above there are several non-Irish tunes.)
Not necessarily. There’s always the warhorse factor, the shunning of overplayed tunes. Players always bring in new, seldom played tunes to keep a session fresh and challenging. Only a small handful of the tunes listed above would be heard at any given session around here.
Of course, once a tune is shunned as overplayed, it soon becomes underplayed. 
Which opens the possibility of reintroducing it to jaded ears with a fresh perspective. And it’s true that regular session players could whip out any of the above tunes.
BTW … What is it with Tam Lin and The Masons’s Apron? They turn up on most “beginners” session lists, but are horrible session tunes! Maybe they look easy on paper, but they’re extremely difficult to execute well. And they’re fiddle showcase tunes that leave other players twiddling their thumbs. Uck.
Paddy on the Landfill? 
(Yes, I found it … American tune, recorded by Wild Asparagus.)
Well this goes to the fact that in sessions there are multiple repertoires, each specific to each instrument, which players of other instruments may choose not to join in on.
There seems to be a group of tunes shared by players of the fiddle, the tenor banjo, and the accordion which are somewhat unaccessable to players of whistle, flute, and uilleann pipes because they dwell below bottom D and/or are in keys with more than two sharps or less than one sharp.
I myself take special delight in playing some of these tunes using various whistles to get the right range or scale, but in my experience most pipers and fluters, when the fiddler/accordion/banjo crowd launch into a series of reels in G minor or A major, just take a break and get another round or go outside for a smoke or whatever.
Yes, sure. But that’s not exactly what I mean here.
Both tunes are all scales-and-arpeggios boring if played as “dots on paper”. They’re really more tour-de-force ponies for individual variation and ornamentation. But that takes difficult and advanced skill, and is especially not conducive to group playing. So I question why these two in particular always seem to show up inappropriately on beginners’ session tune lists. I don’t think I’ve ever heard them played in a “beginner” session where the result didn’t resemble either a fingering exercise or a train wreck.
Do you mean “racing bikes” or is that the tour-de-france?