Can I determine the current IP from a known MAC Address?

I don't think there is a single command to do this. One hack would be to do a ping scan or a broadcast ping on the subnet and then query the arp table for the IP address of the MAC address. Obviously not an ideal solution. Example:

nmap -sP 192.168.1.0/24 >/dev/null && arp -an | grep <mac address here> | awk '{print $2}' | sed 's/[()]//g'

Here nmap will do a ping scan and populate your arp cache. Once the scan is done, the arp command can be used to print the arp table and then you pull out the IP address with grep/awk. You could try replacing nmap with a broadcast ping, but that probably isn't as reliable.

I would simply use

ip neighbor | grep -i "00:1E:C9:56:3C:8E" | cut -d" " -f1