You can always get up and pretend to be moving to some other part of the Carriage/Luas/Bus and hope that the seat is taken by the elderly/pregnant/tired/otherwise deserving person and not by some other able bodied punter. If you don't say anything you risk not getting somebodies back up, just get the hell out.
Personally, if I give someone the seat I don't like to be anywhere near the area coz you have to put up with the gooing and gaaing of aul wans and their "isn't he great/gentleman" stuff. You don't give them the seat because you are some sort of a nice person. You give them the seat so that they don't fall on top of you or someone else. i.e. avoiding a mess that will need to be cleaned up and will delay your journey.
Note: I'm not actually a nice person
