A welcoming candle:
A Red Christmas candle in the window, still popular in Ireland and also in the US, was long displayed as a symbol of hospitality. A tradition in Dublin was to use a blue candle.

Window candles in Ireland were a symbol that the homeowner would welcome the Holy Family – unlike the inn keeper in Bethlehem who bore the guilt of having turned them away.  During times of intolerance for Catholicism in Ireland, window candles also were meant to announce that it was safe to say mass in a home.