You'll need to sign up with Pushbullet by visiting their site @ https://www.pushbullet.com and clicking one of the Sign up with Google/Facebook buttons.
Once you've signed in, on the left-hand side, click on Settings -> Account and on the right-hand side you'll see your access token (You might have to click "Create Access Token") - copy/paste this somewhere for now.
To create a channel, click Channels at the top, then click on "Click here to learn more" in the grey block near the top - they don't make it obvious where to go to create a new channel.
Enter details here - description is optional. Take note of the channel tag you've chosen as it, along with your Access Token, are needed for the Notification Group configuration in the next step.
In IRIS, go to Configuration -> Monitoring -> Edit Notification Groups and click the + in the top right-hand corner to create a new Notification Group. The Group Name, Description and Notification Hours fields are entirely up to you as to how you set them. The View field can be configured to limit which devices will generate alarms for this Notification Group.
Once those are configured, click on Pushbullet below, and fill in the Access Token and Notify Channel fields with the values retrieved previously when signing up and creating your channels.