You suspect that a new ethernet card might be conflicting with another device. Which file should you check within the /proc tree to learn which IRQs are being used by which kernel drives?

Answer: interrupts

Explanation:
The Linux /proc Directory is a Virtual Filesystem provided by linux kernel. /proc contains files and directories that let system administrators and programmers access system information. The file /proc/interrupts file contains information on interrupts and IRQs.

Example:
$ cat /proc/interrupts
CPU0 CPU1
0: 25 14 IO-APIC-edge timer
1: 4710 1831 IO-APIC-edge i8042
4: 0 2 IO-APIC-edge
6: 0 5 IO-APIC-edge floppy
7: 1 0 IO-APIC-edge parport0
8: 0 1 IO-APIC-edge rtc0
9: 0 0 IO-APIC-fasteoi acpi
12: 2493 219349 IO-APIC-edge i8042
14: 0 0 IO-APIC-edge pata_amd
15: 133 25008 IO-APIC-edge pata_amd
16: 136945 7434 IO-APIC-fasteoi nvidia
17: 0 1172 IO-APIC-fasteoi eth0_rename
20: 0 0 IO-APIC-fasteoi ohci_hcd:usb2
21: 701 2368 IO-APIC-fasteoi ehci_hcd:usb1
22: 4 412 IO-APIC-fasteoi sata_nv, HDA Intel
23: 90839 14138 IO-APIC-fasteoi sata_nv, eth4
NMI: 0 0 Non-maskable interrupts
LOC: 236626 98439 Local timer interrupts
SPU: 0 0 Spurious interrupts
PMI: 0 0 Performance monitoring interrupts
PND: 0 0 Performance pending work
RES: 1205807 1212171 Rescheduling interrupts
CAL: 1078 1174 Function call interrupts
TLB: 87901 91058 TLB shootdowns
TRM: 0 0 Thermal event interrupts
THR: 0 0 Threshold APIC interrupts
MCE: 0 0 Machine check exceptions
MCP: 8 8 Machine check polls
ERR: 1
MIS: 0