Irelia is still viable. It's just that a number of the guides on her are out of date and when people follow them to the letter, they do not end up doing so well.
Some general tips on Irelia are:
• Irelia needs to farm a fair bit in order to complete her build. So you will need to last hit very reliably in early game. If you are being denied or harassed, switching to gold items can help.
• Irelia excels in mid-game where the true damage of her W shines, try to utilise this part of the game to gain an advantage for your team. Do not always activate W when you want to engage, since it tips off the other side that you are going to attack.
• W is also the reason that Irelia can get away with being tanky.
• Irelia’s R has a friendly CD and should be used liberally, rather than conservatively.
• Irelia should not usually initiate fights, because in team fights, her priority targets are the carries.
• Irelia’s passive can sometimes be used to detect impending gank or hidden characters, but it is not so reliable.
Hope these help.