I think this is related to
JENKINS-15765 and JENKINS-35179 "IRC Bot does not take commands" : mine does not either, as of release ircbot-2.30 and instant-messaging-1.35
Digging in code, I see that both IRCPrivateChat.java and IRCChannel.java define `getNickName(String senderId)` and `getIMId(String senderId)` routines to implement the interface from https://github.com/jenkinsci/instant-messaging-plugin/blob/master/src/main/java/hudson/plugins/im/IMChat.java#L18 (which says that `senderId` is "the fully qualified IM id of the sender (e.g. for Jabber the user, the server domain and optional resource part)", and one "Translates the sender into a nickname which can be used to informally address the sender." while another "Translates the sender into a unique IM id.") and are used by https://github.com/jenkinsci/instant-messaging-plugin/blob/master/src/main/java/hudson/plugins/im/bot/Bot.java#L162 `getSender()`.
All of these implementations for IRCbot just return the passed `senderId` value, and do not make use of the configurable "IRC Nick" to map the Jenkins account name (which may be privileged to run commands like `build`) to the Nickname of this user on the IRC server, which are two independent accounts.
Actually, looking at the descriptions in the source, it is not evident to me that either `nick` or `id` in the `Sender` intend to mean the Jenkins user account name; but if in practice one does - it is not known to the messaging backend.
The configurable value in Jenkins user account settings ("Your IRC Nick") is only referenced in ircbot/IrcPublisher.java routine `getConfiguredIMId()` (and managed in IrcUserProperty.java), and in https://github.com/jenkinsci/instant-messaging-plugin/blob/master/src/main/java/hudson/plugins/im/IMPublisher.java#L635 `calculateIMTarget()` which seems only used for notifications (e.g. "hey you broke the build") but not for build and such permissions.
At https://github.com/jenkinsci/instant-messaging-plugin/blob/master/src/main/java/hudson/plugins/im/bot/BuildCommand.java#L195 it apparently checks whether the user who sent a command (sender) may build the project in question, but the check is for `project.hasPermission(Item.BUILD)` so no sender here. The docs are a bit vague for me, e.g. https://javadoc.jenkins-ci.org/hudson/security/AccessControlled.html#hasPermission-hudson.security.Permission- says that the command is a "Convenient short-cut for getACL().hasPermission(permission)" where it means probably getACL() for the `project`; whereas items like a similar(-looking) https://javadoc.jenkins-ci.org/hudson/model/AbstractProject.html#checkAbortPermission-- say that it "Checks the permission to see if the current user can abort this executable" but I have no idea how it determines the "current user" and what one is in the non-webgui context of an IM plugin per se.