I haven't taken Focalin, but I have taken
Ritalin which is similar (though not precisely the same). I found
Ritalin to work well, and I found that I wasn't overly irritable.
That said, any stimulant medication has the potential to cause anxiety and irritability. Focalin is worth a try though, because not only are you going to a slightly different medication (dextromethylphenidate
vs. amphetamine), but Focalin tries to treat you with the chemical equivalent
of a scalpel, as opposed to
Adderall's sledgehammer.
As I mentioned, Focalin is dextromethylphenidate, which is a component
of methylphenidate, which is the med from which
Ritalin is made. The difference is that with Focalin, the other chemical that makes up racemic methylphenidate

(the one sometimes blamed for the worst
of the
side effects) is removed, thus hopefully giving you the best bang for your buck, and reducing the likelyhood
of unwanted
effects. All this,
of course, is theory; the only way to tell how you'll react to a medication is to give it a try.
Sorry if it seems I got overly technical, at the end
of the day, the important things to know are that Focalin is more focused than
Adderall, and is overall a slightly different medication. An excellent thread to peruse should you be really interested in the various meds and how things work, check out http://www.healthboards.com/boards/showthread.php?t=698162 , some
of us discuss many
of the meds at length here, and on page two I do my best to explain my layman's understanding
of the technical terms I used above.