Appropriate power supply needed to power 15x 850nm IR LEDs
Definitely put them in series, this guarantees they'll all draw the same current.  You'll need a power supply capable of 33V and 700mA.  Power supplies designed specifically as LED drivers are constant current rather than constant voltage, although they still have a maximum voltage, often around 40V.  You should be able to get an off-the-shelf driver from your favourite LED lighting supplier.