What does Macbeth say the bloody cousins are up to?

"Our bloody cousins" is a reference to Malcolm and Donalbain, who are Macbeth's cousins and who are suspected of having murdered their father, King Duncan.

"bestowed in England and Ireland" means that they have fled the country (Scotland) knowing that they will be blamed for Duncan's death, and are currently living in England and Ireland.

"not confessing their parricide": parricide means the murder of a parent (in this case, Duncan).

"filling their hearers with strange invention" basically means that Macbeth is accusing Malcolm and Donalbain of making up lies to anyone who will listen.

Macbeth has good reason to try to make people believe that Malcolm and Donalbain are lying murderers; namely, that Macbeth himself was the one who murdered Duncan.