But, in an insight into her fragile mental state, she had returned to her old self the following month, looking healthy and with her eyes 'sparkling', he wrote.
O'Riordan was diagnosed with bipolar disorder in 2015, and has also spoken of struggling with eating disorders, alcoholism, and living with abuse carried out when she was 'a little girl'.
Police today confirmed her death at the London's Park Lane Hilton yesterday was 'not suspicious' and is now being looked at by the coroner.
Despite her previous struggles with depression, a producer who was one of the last people to hear from her said she was joking on the phone not long before she was discovered dead.
Local parish priest Father James Walton said the family of the Cranberries singer were 'clueless and bewildered' as to the circumstances of her death.
Father James of St Alibe's Church, where Delores prayed and is to be buried alongside her father's grave, said her siblings and mother Eileen were 'devastated.'
He added: 'They have no idea how Delores died. They received a phone call with the news and are in shock. She was here only last week.
'I visited the family and prayed with two of her brothers. I met Delores when she came to church with her mother.
'There were no airs or graces about her being a celebrity and all that. 'She was a perfectly nice woman and blended into the church.'
The 46-year-old was in London to record a cover of Cranberries hit Zombie with hard rock band Bad Wolves and phoned record label boss Dan Waite on Sunday night.
Mr Waite, managing director of record company Eleven Seven Music, said in a statement yesterday: 'The news that my friend Dolores has passed deeply shocked me. I worked with the Cranberries at Universal Records and have kept in touch ever since.
'Dolores left me a voice message just after midnight last night stating how much she loved Bad Wolves' version of Zombie. She was looking forward to seeing me in the studio and recording vocals.
'She sounded full of life, was joking and excited to see me and my wife this week. The news of her passing is devastating and my thoughts are with Don her ex-husband, her children and her mother.'
His comments chime with those made by Dave Davies, of The Kinks, who said O'Riordan 'seemed happy and well' when he spoke to her before Christmas.
Record producer Martin 'Youth' Glove, who was due to meet Dolores for a recording session the day of her death, said she had been looking forward to it.
'Well I spoke to her at the weekend, we were confirming that she was coming in Sunday night and that she'd come here about midday, 12:30 on Monday,' Glove told RTE News.
'It was good timing for her, it wasn't going to be stressful. She was coming in the night before and we were both excited about seeing each other again.'
Glove had sent her a text on Monday to see if she was still available to come the recording studio, but never got a response. I kind of actually tidied up the home a bit to get it ready for her.
'I thought, actually I'll go out and get some fresh flowers for her, spruce up the studio a bit, she's probably a bit tired after the flight or something,' he said, adding that when he was in the flower shop, he received a call from his manager to tell him that Dolores had died.
Speaking about her musical talent, he said: 'She's a great writer, full stop and that the bottom line.
'Not only that, she had something to say about many issues, including politics and she wasn't afraid to say it, and she said it in a very poetic way.'
Glove claimed that these qualities, along with her 'utterly breathtakingly beautiful unique voice', made her a great artist.
'Not only that, when she came up in the early 90s, there weren't many female fronted rock groups that were edgy and had something to say and weren't afraid to say it, so that made her quite special,' he said, describing her as a 'small little thing' with 'this big voice'.
'You only realise how special she is now she's gone.'
