that's a good point.
however, Irene already gave away her location from her use of the Quick Sword, so this is no longer a reason to not use her regeneration to create a human arm.
irene explains why though:
Irene sees her arms as tools/weapons for combat. As she never learned to use her arms/hands for anything else except wielding a claymore sword and killing Yomas, lol.
So, Irene wanted to die a "human", like Teresa due to jealousy of seeing Teresa "human like" with little human Clare so long ago. Though Irene didn't have a human companion, she was content to live out her life peacefully as a "human".
(there's a funny fan pic, of armless Irene staring at a glass of wine or whatever, trying to figure out how to drink it.... hehe)