I vaguely remember from the dim and distant past that during a game of tig or tag (I think the name varies depending on where you're from) Barley being a safe area where you could not be tigged. I think in my schoolyard the steps where often defined as barley so you could run to the steps shout Barley and be safe. At the beginning of the game Barley areas were defined hence "Barley on steps". Not sure why the word Barley was used but maybe someone else can help?
I read somewhere that it is a corruption of the French word parlez. Seeing that it comes from the verb to speak it does sound sensible. Different words are used in different areas.