Freed from English threats, Scotland's armies could now invade northern England. Bruce also drove back a subsequent English expedition north of the border and launched raids into Yorkshire and Lancashire. Buoyed by his military successes, Robert also sent his brother Edward to invade Ireland in 1315, in an attempt to assist the Irish lords in repelling English incursions in their kingdoms and to regain all the lands they had lost to the Crown (having received a reply to offers of assistance from Domhnall Ó Néill, king of Tír Eoghain), and to open a second front in the continuing wars with England. Edward was even crowned as High King of Ireland in 1316. Robert later went there with another army to assist his brother.

Initially, the Scot-Irish army seemed unstoppable as they defeated the English again and again and levelled their towns. However, the Scots failed to win over the non-Ulster chiefs or to make any other significant gains in the south of the island, where people couldn't see the difference between English and Scottish occupation. This was because a famine struck Ireland and the army struggled to sustain itself. They resorted to pillaging and razing entire settlements as they searched for supplies, regardless of whether they were English or Irish. Eventually it was defeated when Edward Bruce was killed at the Battle of Faughart. The Irish Annals of the period described the defeat of the Bruces by the English as one of the greatest things ever done for the Irish nation due to the fact it brought an end to the famine and pillaging wrought upon the Irish by both the Scots and the English.