That's really too abstract a question to answer concretely, but I'd ask first how do you know it's by the first architect and how do you know it's been mis-attributed to the second? Is it historic or contemporary? Irish or not? Would drawings exist? Printed sources? Are the printed sources/secondary material wrong too?
Then there's the simple question of style. Is it recognisably by one or the other? Or is it one architect working in the style of the other?
Sorry for all the questions, but that's all I can say without knowing more. Though these are the types of questions you should be asking anyway to get to the truth. The truth? You want the truth? You can't handle the truth! (etc. etc.)