Hi i just bought irelia but i need some explanation first due to the fact that she has 2 ability that scales with ap so im wondering if i need to get ap items or something coz i see most irelia playing ad bruiser and secondly i wanna know what the point of her ultimate its kind of sucks except for claring a minion wave. So what should i build when playing her solotop which champ am i good agaisnt who are her counters and how to use her skills effectively?

Irelia is an AD bruiser with high sustain, and multiple damage types which makes it near imposable to itemize against.
Her AP scaling isn't that great, and I've tried building items like a rageblade or hextech on her but with little success. Your biggest source of damage is from your W, and the fact that she can jump into a fight, ignore the CC, and maul some squishies. Her Q is a great farming tool, and your gap closer. Early levels, I don't use it much outside of last hitting with it because the cooldown is so long. Her W is what makes her scary. Passive life steal, and true damage. Build a wits end, and you've got physical, true, and magic damage coming from just your auto attacks. Their tanks wont know what to do. After leveling your w, your e is your focus for the duration, but put some levels into q as well for the cooldown reduction. Nothing worse than someone flashing away and you're 3 or 4 seconds off having that gap closer to finish them off.
Her ult does do a good bit of damage, gives you a lot of sustainability, and gives you procs on your sheen. All of that on a short enough cooldown that you don't have to worry about clearing a creepwave with it. I can't tell you how many times it's kept me alive while I was running away with low health. Throw a blade when they aligned up, and smile when you suddenly have 100 or so more health.

Irelia does have 2 AP scaling abilities but you really don't want to build her like that. She is a very sustainable assassin and should be built like one. Most people go tanky AD because the AD scales with her Q (her initiate/assassinate type ability). The tankyness+W+R lets you live forever too. If you have a sheen/tforce your Q will hit the target, proc sheen/tforce, you'll autoattack doing dmg from the sheen proc+W true dmg. This adds up pretty quickly.

Your R is really amazing actually. If you send it through 2+ champs in a team fight consistently you will regain tons of hp. Also each of the 4 "casts" will proc that sheen/tforce allowing for your auto attack dmg to increase for ~4-5 auto attacks.

she usually is played as a tanky AD or as hybrid depending on how you like to play her

AP Irelia used to be somewhat viable, but then they nerfed her Equilibrium Strike scaling from 1 to 0.5. Don't bother stacking ap. TriForce synergizes well with her.

Her ultimate is her only ranged attack, good for catching someone with low health if your dash is on cooldown. It's great for team fights as you can just send it through everyone and heal yourself up.

As for her build, I usually start with a regrowth pendant, 1 health pot. If it's a heavy ad top (like Tryn or Renekton), I go cloth, 5 health pot. Rush a Philo stone, then get either Merc Treads or Ninja Tabi (situational). After that, get your sheen then Triforce. That is your corebuild. I usually get a warmogs after triforce, but there are other options to go for too.

Incase you're wondering about skill order, I get my Q at level 1 just for easier farming, then I max out W, getting E at level 4. After maxing out W (and getting R at 6, 11, 16), it really depends on play style and situation. I like to max out my Q second so the cool down isn't so long when I go into team fights, but maxing out E is better if you're doing a lot of 1v1 or if you need to cc someone hard.

TL;DR

To sum up everything, don't build Irelia AP, build her tanky dps. Ult heals and is a ranged attack. Rush a Philo, then get Merc Treads/Ninja Tabi, then get Sheen ---->Triforce. Get Q at level 1, then max W, getting E at level 4. Max E or Q second, depending on your play style.

Most people here already summed it up but to be fair, no one goes Trinity Force anymore. There are so many better choices IMO
Also.. Don't use Q to farm if you can just auto attack.