This book - Granuaile, The Pirate Queen by John & Fatti Burke - is part of the Little Library series from Gill Books 

Discover the rebel girl that was Granuaile! 

Granuaile (Gráinne O'Malley) was the daughter of a great trader and sea captain, Eoghan O'Malley of Mayo, 500 years ago. They lived in a castle in Clew Bay and Granuaile spent her childhood sailing in boats of all shapes and sizes. Granuaile's parents wanted her to be a lady and get married, but Granuaile had other ideas. 

So she cut off her hair and set out to sea ...