When does satire become libellous?

From watching the ongoing debate over various figures in Irish political ciricles issuing threats against others for potentially libellous material, when does satire cross the line and become libellous? Satire is the ridiculing of some sort of ridiculous behaviour by others through the use of humour and has been over the years been seen in the public eye from Punch magazine in the 19th Century to such recent programmes as Scratch Saturday or Bull Island to name but a few. Where is the line drawn though with regards to satire and libel laws? The current laws on libel (or defamation as it has now become) are so open ended that technically a lot of satire could be classed as injuring a persons reputation.