for sniffing purposes I need to mirror traffic on an HP 2915. How would I go about doing so?
You can certainly use Port Mirroring with the HP 2915. You can find the information for port mirroring on page B-23 in the documentation for the 2915.
You will be able to designate monitoring of inbound and outbound traffic on either individual ports, groups of ports, static port trunks, and one static VLAN.
An example to assign a port as the monitoring port:
ProCurve(config)# mirror-port 8
Removing port 8 as the monitoring port:
ProCurve(config)# no mirror-port
Once you assign the monitor port you can then select the source interfaces:
ProCurve(config)# int 1-4, monitor
To show the current monitor (this will display what port is the monitor port, called the mirror port, and what the sources being monitored are)
ProCurve(config)# show monitor
A better term would be "port mirroring". This concept is generally universal among vendor products. (Cisco would call theirs SPAN, but it's essentially the same thing). You take a range of ports or VLANs, and simply copy all traffic to a destination port, where you'd hook up a PC with wireshark.
Try this thread - it's not a complete exhaustive walkthrough but it will point you in the right direction. Regardless, searching for terms like "HP Port Mirroring" should yield better results.