This is a Saint Brigid's Cross. Saint Brigid is one of the (Catholic) patron saints of Ireland and her feast day is celebrated on the 1st February,
these crosses were traditionally made using rushes on her feast day. The symbol is thought to pre-date Christianity in Ireland and generally believed to be a Pagan sun symbol and also the symbol of the Goddess Brigid, a Pagan goddess in Irish mythology. Homes of both religious and non-religious people alike, particularly in rural Ireland, still display these crosses as they are believed to ward off house fires.
Biggest thank you to Eirú Ward from Ireland.