iOS App That Delivers Push Notifications for Twitter Hanshtags
iOS apps don't work to filter based on keyword, so you would need an app that has a server side component that reads your stream continually and then sends a push notification to your device.

I would start with IFTTT - https://ifttt.com/ and then look at Pushover - https://pushover.net - if the former doesn't meet your tastes or needs.