I'm in the US visiting family (from Ireland). My 90 days comes to an end in about a week. Can I cross the border (Tijuana), have lunch and return - getting my passport stamped, letting me stay for another 90 days?
You can apply for a new 90 day period of admission by visiting a farther destination (say, Costa Rica) and returning to the US. However, such a move invites suspicion, and it's possible that you would be denied entry. The Visa Waiver Program is intended for temporary visits for business and tourism purposes, and long repeated visits will give the impression that you are instead living in the United States.
You could break your travel into segments, returning home (or going elsewhere) for some time before returning to the US, or, if your circumstances allow, apply for a US visa, which will allow stays up to six months at a time with the possibility of applying for an extension.