Petty, one of my heroes. Ran away to sea at 13 years of age, blind as a bat, jumped ship in France (or thrown off by his fellow sailors), learnt the language and qualified as a physician there by the age of 19, ended up at age 21 with Cromwell in Ireland as the chief doctor of the invasion force, found he loved the place and wound up settling there, refused to be knighted or receive any state pension while he instead made a living from inventing agricultural machinery, refining husbandry of farm animals and occasionally just doing his own thing - like building a galleon sized catamaran (twice). He also helped found two Royal Societies for the advancement of science, and twice defended his position as president of the Irish one in a duel (remember, he couldn't see without bottle glass spectacles). What a lad.
But I digress. "Chargeable" was, by inference, to the crown and simply meant "trusted by and answerable to". In other words he was saying to the newly restored regime of Charles II who were taking over the enterprise from his one-time employer's old crew not to bother thinking they could trust militia raised from native stock until all the Irish DNA had been watered down sufficiently with English blood. In the meantime, be prepared to pay through the nose for an army there, and be prepared to do so for a very long time indeed if they really wanted to hold onto the place. He was, of course, right.