Who said that Syren would be in a relationship with Deadpool? And Saoirse's age shouldn't be an issue since no one at Fox can agree upon a consistent timeline because of this First Class continuity. As for her physical type... well, I haven't seen Saoirse in anything that could give me a real gauge on that.
I thought she did a good job as a special-ops trained girl in Hanna. If she hit the gym then perhaps she could do the job just like Anne Hathaway did as Catwoman in The Dark Knight Rises, so who knows? But I agree, they should cast an Irish actress regardless. Making Banshee American was stupid because it ignores the entire reason he's called "banshee." If he were American they'd just call him "sonic" or "shockwave." Look at Black Canary! She has the same power and she doesn't call herself a character from Irish mythology.