For the second time in four days, Giovanni Trapattoni's side - who were denied their own trip to South Africa by Thierry Henry's handball - dispatched a side who will take part in the tournament, and this time they did so with some ease.
They went ahead with 32 minutes gone when Paul Green, starting for his country for the first time, dived to head home Liam Lawrence's delivery, and Keane extended the lead seven minutes after the restart.
The skipper's 85th-minute penalty completed the job as England's second World Cup opponents, admittedly without several of their bigger names, succumbed all too easily.
Fabio Capello's trusted lieutenant Franco Baldini was among a boisterous crowd, swelled by thousand of Algerians, at the Royal Dublin Showground as he ran the rule over the Algerians.
What he will have learned is debatable as the North Africans were well beaten on the night.
At times they were adept on the ball with holding midfielder Mehdi Lacen and Portsmouth's Nadir Belhadj particularly influential, but they struggled to supply anything of note to their strikers and were woeful at the back.
That may have been just as well for Ireland, with the imposing figure of Rafik Djebbour looking more than capable of ruffling a few feathers.
However, he was dealt with comfortably by Sean St Ledger and, until he limped off nine minutes before the break, John O'Shea.
Indeed, just as they had done against Paraguay on Tuesday evening, it was the Republic who created the better chances despite seeing less of the ball.
They might have been ahead as early as the third minute when Kevin Doyle got across his marker to meet Damien Duff's free-kick with a steered volley, although he was only able to guide the ball into the waiting arms of goalkeeper Faouzi Caouchi.
Duff fired well wide from the edge of the box with 22 minutes gone, but at the other end Keiren Westwood did not have a single save to make.
Green did not look out of place and made one vital interception inside his own box to cut out a Yazid Mansouri cross, but his evening was to get even better.
He bravely dived in to meet Lawrence's 32nd-minute free-kick in front of Caouchi and open the scoring, although he took a boot in the head from the goalkeeper in the process.
However, he was able to continue after treatment and Trapattoni's men returned for the final 45 minutes of an international season determined to end it with a flourish.
They might have increased their lead within four minutes of the restart had debutant Djamel Mesbah not got in a fine block to deny Lawrence.
However, Algeria contributed to their own downfall three minutes later when, after Wolves defender Aldane Guedioura had allowed Duff to cross from the left, Caouchi punched the ball ineffectually towards Keane, who hooked it back over him and into the empty net.
Westwood was called upon for the first time to keep out Karim Ziani's long-range effort with 55 minutes gone, but Ireland were dominating in possession and chances.
Three times inside seven minutes Keane could have added to his tally, first forcing a good diving save from Caouchi before being denied by the post after running onto Kevin Doyle's flick.
He then only just failed to beat substitute goalkeeper Rais Mbolhi with a cheeky 70th-minute lob.
Guedioura came close to pulling one back for the visitors when he rattled the crossbar with a late header, but Mesbah was adjudged to have tripped Keane when he played the ball first and the Tottenham striker stroked home the 43rd goal of his international career from the spot.