You already have the font you need: Lucida Grande. (Certain others on your system may also work, however, if you want a serif variation.) Here's an example of a Gaelic word spelled in the old style:
ḃfuil
That probably will look wrong on Windows (surprise surprise), but if you're using Mac OS X you should see a b with a dot above it. I got this by using Mac OS X's built in character palette, available by turning it on under (of all places) International → Input Menu in System Preferences. Then just find the character (its Unicode value is 0x1E03, if that helps any) and double-click it to insert it into the current (Cocoa) application. TextEdit works well for this; Photoshop and Microsoft Word do not. Lucida Grande appears to be the only font on my system that includes that character, but you might be able to find others online... just search for "Unicode fonts".