What does lace curtain Irish mean?

Answer 1

My mother used it to categorize people that had fantastic looking 'outsides' but were hiding ugly secrets. Those 'lace curtains' were meant to make a family look good, while inside there was child abuse, spousal abuse, infidelities, and so on.

Answer 2

The 'lace curtain Irish' were poor immigrants who had designs on becoming more upwardly mobile. In other words, they were considered social climbers by the other Irish immigrants.