Is Irelia OP?

No.

She's not overpowered, but she is a good choice against the current meta picks. She's in a similar position as Trundle was at the beginning of Season 4 (where tanks were overpowered) since she's a generally good pick who happens to counter the current meta top champions.

She works well against ranged, squishy champions which are currently popular in the top lane. Additionally, she is a good choice against champions who depend on their CC for protection.

Basically, she's a strong pick into champions such as Gnar, Kennen, Lissandra, etc. Since she can hold her own against many bruisers (she might not have an advantage, but she can farm and hold her own), she's a pretty safe pick against most champions.

No.

I don't think so. She's definitely strong. She's good at turning fights. But there is counterplay to her, and she does have weaknesses you can exploit.

The main thing some people have brought up lately is her stun, but that actually requires you to take an unfavorable trade in order to proc it.

The other thing is her tenacity. Honestly, I think now that it only stacks additively (as of a few patches ago), it's not too awful. Also, it doesn't work on knockups.

But man, if you knock up my Irelia, I will find you.