For Irish Pubs you don't exactly have to go in there with a 3-piece
I am no fashion expert, but if she normally sees you in the attire you mentioned, I would have to say just going with some clean lined pants, such as the flat-front slacks mentioned earlier in this thread, and a tighter (not skin tight, but not a loose t-shirt) solid-colour shirt should give her the "I can be suave too" look, without out-dressing her (which is a very important point to consider). If you find a short-sleeved shirt with a very slight v-neck it will accentuate your shoulders more, lending more style than a simple ring-neck, and give a more masculine look.