Eoghan O'Mahoney (34), of Craigford Drive, Killester has pleaded not guilty to the attempted murder of his grandmother Catherine McGillion (91) on December 19, 2013, by reason of insanity.
Ms McGillion was cut with a knife and repeatedly kicked in the body and head during what was described as a horrifying ordeal.
Counsel for the prosecution Pat McGrath SC said the plea of insanity is not contested and read a statement made by the elderly woman in which she described the close relationship she had with her grandson.
The accused man's brother Conor O'Mahoney took the stand to say that their grandmother is the "person he loves most in the world" and that he "loves her to bits".
The court heard from two psychiatrists who both agreed that Mr O'Mahoney was suffering psychotic delusions when he carried out the attack and should be found not guilty by reason of insanity.
Justice Patrick McCarthy will finish charging the jury this afternoon.