Do you get issued new uniforms when reenlisting from the IRR into active duty?

Kinda yes. When you get to your Active Duty unit...and if you have no uniforms by the time you get there, ask your Commander for a memo that you can take to Clothing and Sales and you should get at least 1-2 sets of duty uniforms. The rest will come via Clothing Allowance.

Not sure about enlisting, but those called up from IRR are issued the style fatigues in use at time of call up.

I had a break on Service for 18 months and when I got back to AD I showed up in civilians until the CDR provide me a Memo to get new uniforms, I've got all the basic issue uniforms including dress greens at that time.