Celtic calligraphy made great use of lettering made from fantastic twisted animal shapes, or richly decorated with spirals, knotworks or key patterns. Below are a few simple examples of Celtic decorated capital letters.

The basic font upon which Celtic lettering is based is given below.

Sabhal Mór Ostaig, a Gaelic medium college on the isle of Skye in Scotland has a number of Celtic fonts for Windows available for download including the one above which is called Bunchló. Follow the link below to download fonts.
http://www.smo.uhi.ac.uk/~smacsuib/fonts/