In active DCC, your client connects to the remote client that is serving the file.
In passive DCC, your client will open a port that the remote client connects to to send the file.

Passive DCC support landed in irssi in v0.8.10.
https://raw.githubusercontent.com/irssi/irssi/0.8.10/NEWS

Here’s how you enable it:

1.a) set dcc_port in irssi to the port range you will be using
example: /set dcc_port 12345 12347

1.b) forward these same ports in your router to your irssi client machine

2.a) set dcc_own_ip in irssi to your external ip
example: /set dcc_own_ip 11.22.33.44

2.b) if you’re using a NAT setup in linux, set net.ipv4.ip_nonlocal_bind = 1 in /etc/sysctl.conf

Not sure if this is also needed: /set dcc_autoaccept_lowports ON

Edit:

It looks like irssi can’t handle changing automatically between passive and active DCC.
So you need to run /unset dcc_own_ip and /set dcc_port 0 to go back to using active DCC.

irssi aliases to help with that:

passive dcc: /alias pdcc /set dcc_own_ip 11.22.33.44;/set dcc_port 12345 12347
active dcc: /alias adcc /unset dcc_own_ip;/set dcc_port 0

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