This is the flag of Scotland and is often seen flying in areas where there was heavy Scottish immigration in Northern Ireland.
The X shape of the cross, also called a saltire, is based on a belief that Andrew the disciple of Jesus (Saint Andrew) was crucified on such a cross. The symbol was used by numerous Byzantine emperors.
Who was St.Andrew?
St Andrew, brother of St Peter, was a missionary in the area around the Black Sea. He was crucified in Patra, Greece, on an X shaped cross. Legend has it, that some of his relics were taken to Scotland and buried there.
St Andrew has been the patron of Scotland since the 11th century. The White Saltire of St Andrew has been used as a Scottish national symbol since the 12th century. It was only in the 15th century that the blue field was introduced.
As the flag of Scotland it is the oldest national flag still in use the earliest attestation being in AD 832.
Now that you know all about this flag, why don't you read about the other unofficial flags of Northern Ireland?