This feature is available since AndroIRC 3.0. Your device needs to run Android 2.3 or above in order to use FiSH. It's not a limitation from us, but from Google.
Starting with version 3.0, AndroIRC supports FiSH encryption.
You can use the /key command to set the FiSH key (See Command list for more details), and /delkey to remove it.
You can use the /keyex to exchange key with someone else (See Command list for more details). Command available for channels are also available for pv.
Starting with AndroIRC 4.0, you can specify the blowfish algorithm to be used for encryption. Two algorithms are available: ECB and CBC. CBC is more secure, but not widely available.
To set the blowfish algorithm, just prefix the encryption key with either