If you want to post animated gifs and embed youtube videos you can't use IRC. Everything depends on the requirements. To find the right solution you have to understand what your client needs, not what he thinks he needs.

Yes you can, plenty of IRC clients let you post animated gifs, some would let you embed youtube videos I am sure.

Nope. And you have no guarantee that the server supports sending that, or that anyone else is using the same IRC client.

Then the question becomes: Is your company Slack?

A lot of assumptions that I think distract from the important - understanding the need vs solution first thinking.

The business might not be out to build their slack, chat could just be enough of a transformer alone. Besides, Slack is IRC client re-skinned with a bit different rendering. It's not the first either but maybe a business case is good enough to just chat as a big improvement.

Many organizations do just fine using something like Microsoft Lync, as much as I like Slack it's not for all users or instances.

This is a terribly flawed analogy.

90% of companies that try that are going to fail.