Regarding the filing of tax returns & implications, you could begin by browsing through the IRS website
http://www.irs.gov/taxtopics/tc851.html & here
http://www.revenue.ie/en/personal/circumstances/moving/tax-residence.html especially the section regarding Double Taxation Agreements.
Some of the ex-pat forums – which are mainly used by British ex-pats but a lot if not all of the advice & information you would pick up there would also apply to an Irish ex-pat, can also be very useful.
Have a look here: [broken link removed] & you could search through here for threads on tax issues: http://britishexpats.com/forum/forumdisplay.php?f=57
Just as a side issue, I don’t know what sort of immigration arrangements you have made regarding your status when entering the US before you actually marry, but just be aware that if USCIS become aware at your port of entry that you intend to marry a US citizen, & if you are not entering on a Fiancé(e) Visa (K1 Visa) then you could be refused entry on the grounds that you are entering for a different purpose than stated on your present visa (presumably a holiday visa or a non-immigrant temporary work visa).
What’s meant to happen is that your boyfriend is meant to file a Petition for Alien I129F with a USCIS office in the US, & after this has been approved you are meant to complete the application process here in the US Embassy.
In reality many people skip/bypass this process & just file in the US for Adjustment of Status & a Petition for Alien Spouse after they get married in the US. I just wanted to
highlight the above regarding entering the US. Some people use an immigration lawyer to file for Adjustment of Status after getting married, but in reality you can download a package of forms to file concurrently & providing that you submit the correct ones there is no need to go to the added expense, providing that your application is relatively straightforward. A good site/forum for help on this is
http://immihelp.com/