How do you use an identity file with rsync?

Use either $HOME

rsync -avz -e "ssh -p1234  -i \"$HOME/.ssh/1234-identity\"" dir remoteUser@server:

or full path to the key:

rsync -avz -e "ssh -p1234  -i /home/username/.ssh/1234-identity" dir user@server:

Tested with rsync 3.0.9 on Ubuntu