Did much the same myself - moved from SF to Cork 5 years ago (originially from Cork). Would highly reccomend the move - travelling is great but there's nothing like spending a few years in a place - you get under the tourist radar, make local friends etc.
Living abroad is a wonderful idea and something very few people live to regret - quite a few never come back! That's the one warning I'd give - once you've lived abroad it can be hard to settle back in to home. You always want the best bits of all places.
Cost of living differences are a factor - renting an apartment will be cheaper than San Francisco certainly and likely to be cheaper than most of the bay area for that matter. Other costs will be higher - gas, car insurance ( can be a stinker) , utility bills will be a bit more than in the US. Overall it should balance out to be much the same lifestyle on 40K euro as on 40K $.
I'd suggest renting a room in a shared house for a while until you find your feet - plus it's a good way to meet a few people. Try www.daft.ie to get some ideas.
You should be able to bring in your cat - they've relaxed the restrictions in that last few years on pets. You'll have to get the cat microchipped (no big deal my dog had it done and it hardly bothered her at all). See http://www.citizensinformation.ie/categories/moving-country/moving-to-ireland/practical-issues-when-travelling-to-ireland/bringing_pets_to_ireland
for more info. In fact www.citizensinformation.ie has a moving to ireland section that you'd probably find helpful.
For more info on the cat contact
Department of Agriculture in Dublin
Dept.:Special Projects Unit
Floor 4 Centre
Tel:+ 353 1 607 2827
3 years would be about right in my mind - 1 year is way too short and 2 would be okay too.
Cork is a much more cosmopolitan place than it was 12 years ago (sorry previous poster) as there's been a significant influx of imigrants in the last 4-5 years mostly from eastern europe but there's a good scattering of Americans among others.
Hard to answer you on life styles without some context - I guess it'd be up to you what you want to do. The variety of options is there, from loads of outdoorsy stuff to cultural groups, some great food choices, decent pubs etc. Generally speaking Irish people (and Cork people in particular if I may say so) are welcoming and open to pretty much anything. There's a vibrant well established local gay scene for example.
The food is excellent as well with good local markets not just the supermarket fare. The food is much better than the US generally though perhaps not quite as good as the bay area when it comes to fruit and veg. Still miss the pears from the farmers market on the Embarcadero