In order to get the best visual effects on this site, and to have the pleasure of writing in various Celtic styles, please take a look at the St. Patrick's Day package of various Celtic fonts and templates for cards, notices, and more, on Font Marketplace. Click here: http://www.fontmarketplace.com/font/st-patricks-day-font-pack.aspx
The cost is $9.99 to download the package to your computer and it will be automatically placed in your list of fonts in Word, Publisher, etc.
Along with the accents Irish Gaelic uses over many vowels in its words which you can now create with the International Keyboard or Sticky Keys, you will have everything necessary to type those Celtic words in true Celtic scripts.
Check out their other specific occasion packages while on their website.