The Irish Mastiff is not a purebred dog. It is a cross between the Mastiff and the Irish Wolfhound. The best way to determine the temperament of a mixed breed is to look up all breeds in the cross and know you can get any combination of any of the characteristics found in either breed. Not all of these designer hybrid dogs being bred are 50% purebred to 50% purebred. It is very common for breeders to breed multi-generation crosses.
"This is our Irish Mastiff hybrid puppy. His name is Diesel, and he is 6 months old in this picture. He is half Irish Wolfhound and half Mastiff. He is a very well tempered dog, slow to learn but eager to please. He's grown fast since we first got him, but not sure of his exact weight. He was born the day my husband and i were married so we knew he was meant to be in our family."
ACHC, DDKC, DRA, IDCR