I once built tank Soraka because no one on my team wanted to do anything to counteract the other team getting fed. I just walked straight in and took hits for a few seconds while the rest of my team got into a good position and then joined the fight. We actually managed to win that game.
Point is that anyone can become a tank if you build the items. Some champions just have skills that lends themselves to being a tank (damage reduction, heals, etc.). Having said that, tank Irelia really does work. Once you have that trinity force, bladesurging to an enemy generally gets a reaction of "Oh s***!" The stun, true damage, and innate healing ability scare a lot of people, so they're more than happy to focus you.
Basically, tank Irelia works, and I've done it on several occasions. The only downfall (and major downfall at that) of tank Irelia is that she can't peel enemies off of teammates or support them. (Irelia's stun will often turn into a slow simply because the tank maintains a lot of hp while the enemy teams' drops.)