Not necessarily, if they joined the channel before you, and if they haven't typed any channel messages, private messaged you or set any channel modes, then their address won't be in the IAL unless you /who them or the channel, or whois them.
~ Edit ~
From mirc.chm
"A user's address is added to the list either when they join the channel, send a message to a channel, or make a mode change."
If they haven't done any of the above, and you didn't use a server command to get their address, ie. /who /whois /userhost .. then their address may not be in the IAL.
/help /ial
