Does anyone know how to translate strength (as in inner strength) or fortitude into Gaelic? And is there a different way to say it for males and females?

Here's a word that isn't used a great deal, but really sums it up, I think: "gus", which the Collins dicco defines as "courage, grit; initiative; self-assurance". The word goes all the way back to Old Irish sagas, where it is used in the description of warriors. If you go to:

http://quidnunc.net/~garyi/noda/notae.html

you can find the word written in the old manuscript hand, using a scribal contraction for "us". Oh, and "gus" is just "gus" whether it's in a man or woman.

A bit off topic here. But just if anyone wants to see another look at some older writing abbreviations.

http://www.fainne.org/noda/

Má tá spéis agat sna noda, a Mhaidhc, caith súil ar an suíomh seo:

http://www.ucc.ie/faculties/celtic/lss/