Speaking at St Anne’s in Belfast’s Donegall Street, Stephen Lusty said she showed him pictures of the engagement ring.
Ms McKee was shot dead on Thursday while observing rioting that night in Creggan after police carried out searches for dissident explosives and weapons over fears the New IRA was to mount attacks over the Easter period.
She had recently moved to Derry to be with her partner Sara Canning.
President of Ireland Michael D Higgins, Prime Minister Theresa May and Taoiseach Leo Varadkar and other politicians are at the service.
The New IRA said its members killed her.
“She showed me pictures of the ring she had bought for Sara and told me of the fabulous plans she had of her proposal in May,” said Mr Lusty.
“She made me put a date in my diary for the wedding in Donegal in 2022.
“And gave me strict instructions to wear my kilt, which she always wanted to borrow, adorn it with some Harry Potter, and to find or re-find my own version of Sara.”
Ms McKee’s sister, Nicola Corner, told mourners: “In the words of Lyra herself, we must change our own world, one piece at a time.
“Now let’s get to work.”
Many friends of the journalist are wearing Harry Potter-themed T-shirts, scarves and badges to the funeral service at St Anne’s Cathedral. Ms McKee was an avid fan of the series.Speaking at the beginning of the service Dean Stephen Forde said: “Lyra was a person who broke down barriers and reached across boundaries, this was her hallmark in life, this is her legacy in death.
“As a journalist she pursued truth wherever it took her, never content with the sullen silence of unanswered questions.”
He added: “She was a child of the Good Friday agreement. She was a primary school pupil in north Belfast when the agreement was signed .
“She grew up to champion its hope for a society that was free from the prejudices of the past.”
Three people – a 57-year-old woman, and two teenagers aged 18 and 19 – have been arrested over the murder.
All have since been released unconditionally.
Following today’s service, Lyra’s remains were being laid to rest at Carnmoney cemetery in Co Antrim.
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