I’ve dusted off my harmonica playing in the last couple of weeks and I’m really enjoying it, especially trying to play reels and jigs. I know about the tremolo and octave harmonicas in Irish music, but does anyone have any experience with chromatic harmonicas? Specifially: how does the slide affect tuning, and what is/are the best chromatic harmonica key(s) for playing IrTrad. Thanks!
Try this:
Lots of material in this site, and some harmonicas for sale. 
There’s a lot of information, for example, with each of the custom harps that he has for sale.
Glauber
The “slide” is used to sharpen any note you are playing by 1/2 tone and is also used to “trill” notes.
I love my Hohner Chromatic 12 in G major. It works well with A key bagpipes,D key whistles and G key concertinas. If you want to play fast reels you could use a d’ chromatic for higher pitch melody.
I recently purchased an ‘Irish Trad Session Harp’ form Brendan Power in the keys of D and G. Basically it is a customized chromatic hamonica with the spring removed so that when the slide is out it is in D and hen it is pushed in it is in G. It is a wonderful instrument for IT as you can easily switch betwen keys which often happens in IT sessions.