Two questions...

First question: What are the kinds of "scales" or effects that you use in order to make a music or just a melody sound celtic? What chords, rythms?

Second question: Precisely the same doubt as above but in "pirate"-like music, if you get what I mean. That kind of catchy tunes and rythms in pirate songs and dances.

Everytime I try to get something like that from my piano I get stuck and nothing comes out...

Hey! My area of expertise!! There's no set rules for it really, you either can or can't. My suggestion to you is to try to listen to a lot of music that's considered to be within that style and see if you can get the hang of it that way. Otherwise just play around with minor to major chord progressions and 6/8 time signatures. Also try for a lot of syncopated rhythms for the piratey stuff.

Talk to me outside the forum and I can suggest some examples that might help you :D

The only thing I can suggest for Celtic music is some forms of pentatonic scales.