The pair arrive at Ewood Park following centre-half Shane Duffy's departure to Brighton for an undisclosed fee.
Williams, 23, made 131 appearances for the Robins, helping the club to the League One title in 2014-15.
Hoban, 22, has played 58 games for Watford, scoring twice.
Both players, who have Republic of Ireland Under-21 honours, will be available for Saturday's Championship game against Fulham.
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She had been widely tipped as the politician who would take over from the former deputy first minister, who retired partly due to illness.
The 40-year-old County Tyrone woman is the current health minister having previously been agriculture minister.
She paid a warm tribute to Mr McGuinness and said she was "following in the footsteps of a political giant".
She said "no-one can replace Martin" but said she would continue the work he started.
Mrs O'Neill will have just five weeks to prepare for an election after Stormont's power-sharing coalition fell apart over a botched energy scheme scandal.
The fallout from the Renewable Heat Incentive scandal, which is approximately £490m over budget, led to Mr McGuinness' quitting after DUP leader Arlene Foster refused to stand aside as first minster while an investigation was carried out.
As they hold a joint office, his resignation automatically put Mrs Foster out of her job and prompted the calling of snap elections on 2 March.
Mrs O'Neill described her new role as "the biggest honour and privilege of my life".
"I have never been afraid of challenge and I have never been afraid to act," she added.
She referred to her first act as health minister last year, when she ended Northern Ireland's blanket ban on gay men donating blood.
Mr McGuinness said he was "overjoyed" at her appointment, which he said was part of a "generational change" within his party.
He said that as health minister she had already taken on one of Stormont's toughest portfolios, adding that Sinn Féin did not appoint people to such ministerial posts unless they had the "all-round ability to manage very difficult situations".
Tackling mounting hospital waiting lists has been a huge task for Mrs O'Neill since she was appointed health minister last May.
In October, she launched a 10-year plan to transform the health service, saying it would improve a system that was at "breaking point".
Opposition politicians questioned the lack of details in the plan, which was not costed.
But it set out a range of priorities, including a new model of care involving a team of professionals based around GP surgeries.
Mrs O'Neill was first elected to the assembly in Mid Ulster in 2007 and has held various senior positions within Sinn Féin.
She has worked for the party since 1998, initially as political adviser to MP and former MLA Francie Molloy, before being elected to Dungannon and South Tyrone Borough Council in 2005.
In 2011, she was appointed as minister for agriculture and rural development.
The following year, she announced the department would move to a former British army barracks in Ballykelly, County Londonderry.
After the announcement, it came to light that Strabane had been chosen as a more suitable location by an internal DARD assessment, a decision that Mrs O'Neill then overruled.
In February 2013, it was also revealed that the decision had been questioned by then Finance Minister Sammy Wilson.
Blackburn Rovers have signed defender Derrick Williams for an undisclosed fee on a three-year deal from Bristol City, while Tommie Hoban has joined on a season-long loan from Watford.
[NEXT_CONCEPT]
Sinn Féin has announced that Michelle O'Neill is to replace Martin McGuinness as its leader north of the border.