Your father could be of just about any nationality with a name like "McMahon", or with any other kind of name. As for his ancestry, though, one might guess that his people were Irish (at least, on his own father's side of the family).
What really concerns me, though, is why your foster parents won't answer your questions about your heritage. If you're old enough to log onto the 'net and start making intelligent inquiries (as I see you're doing here), then you're certainly old enough (and presumably mature enough) to handle whatever it is that your foster parents feel they've got to "shield" you from.
Have you had a heart-to-heart conversation with them about this? If not, then please do so, and tell them how you feel. If they can't -- or won't -- help you, there are other means of finding out the information you wish to have. Tell them this, too. Let them know that it's a far better thing if the answers you are looking for come from them, than from somebody else.