She is definitely not a weaker Fiora.
Irelia is a Diver. What are Divers, you ask? Champions who like to dive onto the backline and do a shit ton of burst, then sticking around to tank and DPS off a kill or two.
Irelia in particular is a heavily single target diver. Almost all her damage is single target, meaning she's very good at eliminating that carry early on in a fight. There are other divers like Wukong and Jarvan who are AoE, and when not build heavy damage, often cannot kill a target as quickly, however the majority of their damage is also AoE or multitarget.
Irelia also has decently high DPS with ult and W, as well as a fairly low cooldown E. This makes her into an above average duelist, allowing her to splitpush decently (although not amazingly due to lack of waveclear without Titanic) and go head to head with tanks. However, this is not Irelia's main purpose - she's an above average duelist, but not an amazing one. She lacks the tools to fight skirmishers when on an even gold and XP level (although when ahead she can definitely fight them), or even some other divers, who are the ones she'll usually be up against.
Now Fiora is very, very different. She is primarily a splitpusher and is a terrible teamfighter. Trying to get onto a carry with that low range of a dash is absolutely impossible unless facing a terrible team, or unless flanking against a slightly less terrible team.
Even if Fiora gets into a teamfight, she's not looking to tank up any damage, like a Diver is. Her main priority is dealing damage, and her survivability is pretty shit. Putting Irelia and Fiora both in the middle of a teamfight - not only does Fiora have a harder time getting there, and will probably already be half health, she also lacks the defensive stats to survive more than a second.
Fiora is an amazing duelist who can go head to head with almost the entire champion pool. She also wins via statcheck over a large amount of bruisers and tanks, similar to Irelia, although Fiora generally outstatchecks a wider range of enemies. Fiora, however, also does have the tools to fight other skirmishers.