It can be assumed that the British isles were a net exporter of soldiers prior to the Glorious Revolution of 1688. Spain was amongst the many continental powers to hire regiments of English, Irish and Scottish. Some information be found on the (Spanish language) website LA ÉPOCA DE LOS TERCIOS, the age of the tercios. British tercios / regiments are found here. An impressive list!
A quick, and far from complete, survey of colonels mentioned makes me to believe that most obtained permission by the (Stuart) monarch to raise a regiment. It is perhaps not really surprising that many were Catholics.