Simplify the following text for a Grade 7 native English-speaking person.
Original: Marcus Paul Davis ( born August 24 , 1973 ) , nicknamed `` The Irish Hand Grenade '' ( and `` The Celtic Warrior '' when fighting in Ireland ) , is an American mixed martial arts ( MMA ) fighter .
Simplified: Marcus Paul Davis ( born August 24 , 1973 ) , is an Irish-American mixed martial artist .