Who Do You Think You Are? (BBC One, 9pm)

COMEDY and tragedy are never far apart, and in the case of the man who has created the comic character Mrs Agnes Brown, Brendan O'Carroll, the horrific murder of his grandfather – who had the note "Traitor to Ireland – shot by the IRA" pinned to his chest – is at the centre of tonight's Who Do You Think You Are?

Mrs Brown has saved O'Carroll from penury after his foul-mouthed creation shot to fame at a time when he'd lost a fortune trying to lift Ireland's film industry out of the doldrums.

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"Initially it was me playing Mrs Brown. Before you knew it everyone was involved in the show: my wife (Jennifer Gibney, who plays his screen daughter Cathy), my sister, my son... It's really hard to throw a stone at the production without hitting one of the family. Although she can be an awful woman, she loves her family. She would defend them to death," he says.

Love her or loathe her, BBC executives have seen Mrs Brown top the Christmas specials tree for the past two years and then produce that much-valued movie success with Mrs Brown's Boys Da' Movie. Critics didn't raise a glass of Guinness, but it was a top ten ticket-seller – a follow up is now in the pipeline.

"Mrs Brown started as a five-minute piece on radio... mad," explains O'Carroll, who adapted Agnes for an Anjelica Huston movie having started in comedy relatively late, aged 35.

The comedian is determined to find the men who murdered his grandfather Peter O'Carroll, one night in October 1920, at the height of the Irish War of Independence.

"My own family history, I know a bit," he explains. "The stand-out story of the O'Carroll side was the murder of my grandfather in 1920. He was shot and it was for Irish freedom... what happened that night? I saw it on the gravestone – 'shot during the curfew' – it was really stark and I thought, I'd really like to know what happened."

As you'd expect, with the BBC's expert team on the case, the truth emerges and is far from a laughing matter.

DIY: SOS Big Build (BBC1, 8pm)

IN Cumbria Nick Knowles and the team meet Rob and Michelle Wall and their son Noah, who was born with spina bifida, a condition which required a great deal of care and attention from the family. This means the planned renovation of their home, a small 18th Century house, had to be put on hold. Now, they have been left with a property which is both cold and potentially dangerous, and it falls to designer Charlie Luxton to draw up the plans to make it a safe and homely haven for the clan.

As well as calling on the expertise of their own craftsmen, the team find no end of willing volunteers from the Walls' own community, all eager to lend a hand. It's heart-warming stuff.

Rizzoli & Isles (Alibi, 9pm)

THE two women Boston police investigators – following the usual convention of good-cop-bad-lovelife – are back for a fifth series. Angie Harmon plays police detective Jane Rizzoli and Sasha Alexander (who US cop show fans will recall was a shock early casualty in NCIS as Special Agent Kate Todd) is medical examiner Dr Maura Isles. The series is based on the novels by Tess Gerritsen.

Tonight Jane is struggling with pregnancy while trying to catch the killer of a jogger. One complication will be the plot attempting to explain the missing face of Lee Thompson Young, 29, who played detective Barry Frost. He was found dead from what was reported to be a self-inflicted gunshot wound after failing to turn up for filming.