Dockery chose to sing "The Folks on the Hill" — a '30s Hammerstein favourite that meant a lot to the couple, reported the Belfast Telegraph.
The actress said that she did not know whether she should sing or not but revealed that John's father was "just as persuasive as his son."
Alongside Dockery, mourners at Dineen's funeral included his parents Patrick and Marie, his sister Trish and brother Robert and a vast number of extended friends and relatives.
One of Dineen's favourite pieces of music opened the funeral — Van Morrison's "Days Like This." Gifts placed at the altar to symbolise his life included a childhood toy monkey, a Munster rugby jersey, the Norton Anthology of Poetry and one of his work ties.
During her emotional tribute, Dockery described her 34-year-old fiancé, who worked in PR in the London financial sector, as "charming, charismatic, cheeky and considerate."
"John's charm was legendary," she said. "He could persuade anyone to do what he wanted in the least pushy way possible — particularly with women. I was honoured to know John, to love him and to be loved by him. He was my friend, my king, my hero, my everything. We celebrate him, we honour him and we will miss him."
Allen Leech, who plays Tom in Downton Abbey, read the poem "The Summer Day" by Mary Oliver, which ends: "Tell me, what else should I have done? Doesn't everything die at last and too soon? Tell me, what is it you plan to do with your one wild and precious life?"
Dockery had rushed back from a Downton Abbey promotional tour in the U.S. to be with Dineen during his final days. They had been dating for two years after being introduced by Leech and were believed to have got engaged earlier this year.
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