The 27-year-old was the father of Irma's daughter Nikoleta (9).
Earlier this year Irma split from Danny O'Donoghue of The Script.
Mr Simanaitis' death was the subject of an inquest at Dublin County Coroner's Court where a suicide verdict was recorded but later quashed in the High Court following a judicial review brought by his brother Donatas.
At the Coroner's Court Lukas Tiskervirias -- a friend of the deceased man -- said prior to the shooting, which occurred in the early hours of the morning, he had been drinking with Marius.
Mr Tiskervirias said Mr Simanaitis had been showing off the gun earlier in the evening before putting it away.
Mr Simanaitis had been talking about being "cheated" out of a large sum of money which he was due to pass on to someone else and that his "friends could not trust him anymore," he said.
That night, Mr Simanaitis had blocked the door of the apartment with a barbell, drawn the curtains and would not allow anyone to go onto the balcony.
He had also been talking about his ex-partner and child.
The last time Mr Tiskervirias saw Mr Simanaitis, he was sat with his head in his hands.
Mr Tiskervirias said he was lying down on a sofa in the sitting room with his eyes closed.
He said it was very dark in the room.
Mr Simanaitis said he was going to listen to a song for one more time and then he was going to sleep.
"I heard a strange sound, like someone scratching but I didn't pay much attention to it," Mr Tiskervirias said.
After this he heard the shot.
"I sat up and opened my eyes. I didn't switch on the light but I could see Marius lying on the ground with a black spot around him. My friend was with his girlfriend in another room. I was shocked I ran into that room, and I said Marius had shot himself. They didn't believe it until they saw the body.
Mr Tiskervirias told coroner Dr Brian Farrell that he had met Marius at 2pm the previous day at the Leisureplex in Blanchardstown.
He said Marius was in good form and had a rental car which he returned to the garage.
On the way back from dropping the car to the rental place, they bought a bottle of Hennessy. They rang a friend at 9pm that evening to invite him to the house, and he subsequently arrived with his girlfriend.
He knew Marius for a year having met him at the gym.
"He didn't give any warning he was going to shoot himself. I heard the shot and I heard a sound like something had fallen. I opened my eyes and saw Marius," he said.
Deputy State pathologist Dr Michael Curtis told the court the death was due to a gunshot to the right of the head in an area well recognised as a site used by people inflicting wounds on themselves.
Dr Curtis said there was no evidence of assault. The toxicology report found Mr Simanaitis had been mildly intoxicated when he died and that there was methamphetamine in his system. The inquest continues.