I did a search but still a little confused. I have a LeHart keyless flute and am considering to add one or two keys. I play some tunes with friends that are in many keys. If I were to get 2 keys, which would be the most useful?
Personally, I’d just learn to half hole. 
Maybe you would want a natural C & F(?) - it would really depend on the tunes you aim to play, (you might want a Bb & Eb). 
I believe the traditional or most common first two key choices for many people would be F natural (long or short, I’d do long) and G#. That’s what I’d do anyway, to cover the largest number of tunes I play that would benefit. After that I’d want an Eb, and then a Bb that would cover a much smaller number of tunes.
Just my personal preference. I have an 8-key flute and I don’t ever use my C natural key because it’s so easy to cross finger, and I like the slightly sharper pitch compared to the Cnat key. Others might consider the Cnat key more important. Also I don’t ever use my short F key. Your mileage may vary, but I think a G# and long Fnat would be “safe” choices as the first two additions.
If 90%+ Irish trad then C/F/G#.
If you’re playing a reasonable amount of music in other keys hard to prioritise - need em all. But Eb and Bb perhaps on basis v hard to half hole and produce balanced tone.
Definitely will need an ignition key, and a key to the city.
But on the flute, a G# and F nat. All others are sparingly used in most session tunes.
It depends on which notes are used in the kind of music you play. Here’s my opinion based on the flutes that I have and the kind of music I play, but you may find that results vary.
F natural I find impossible to play in tune, so a key is needed here. Although it is manageable on a traverso. Just that most of the flutes used for Irish music are based on 19th century models,
and they don’t cross-finger F natural too well.
B flat is easy to cross-finger. Even when I’m playing on a flute with a Bb key, I never use it.
The high G# cross-fingers pretty well and is easy to play. The low G# can be done, but it is not really in tune. However, for the tunes that I play, a high G# is much more common than a low G#.
And in the tunes that have a low G#, you can generally get away with being not quite in tune because the note doesn’t last too long.
I find E flat impossible to half-hole. But on the other hand, it rarely shows up in the music that I play, so I don’t bother with it.
I guess one thing is whether you want to be a person who has just one flute. Somehow I have ended up not being that person, so if someone really wanted me to play something with an Eb, I would get out a flute that has an Eb key.
If you have people asking you to play chromatic stuff all the time, another option would be to just get a Boehm flute for that. I was in my local woodwind shop today, and they said that a decent used student-grade silver flute can be had for about $150.