In Ireland the red circle signs never mean prohibited, infact the exact opposite! If something's in a red circle it means that it's a regulatory sign and must be obeyed.. e.g. an arrow pointing left in a red circle = keep left. These are gradually being replaced by the standard European white on blue signs to prevent confusion!
Prohibited = Red circle with a line crossed through it.
No Entry (Unique to IRL as far as I know but very logical)
Straight ahead only (one way system):
Normal Yeild sign:
Here's a Yeild sign in an Irish speaking area on the west coast:
These also appear everywhere:
Our guy looks less like he's working with an umbrella:
This one is VERY obvious: (At docks/rivers/quays)