Drag all files from the folder Fonts & Keyboard (except this “Read me” of course) onto your System Folder. The keyboard layout U.S. Celtic lets you type all special Celtic characters with dead keys. The suitcase Celtic Intl Resources does not contain any fonts but the necessary resources that allow you to select Breton, Gaelic, Welsh in the control panels Date & Time, Numbers, Text.
You can get Celtic TrueType and PostScript fonts from:
info-mac/info/convert-celtic-fonts-11.hqx at any Info-Mac mirror