The cash was revealed on Monday, June 26, as details emerged about the deal with the Democratic Unionist Party (DUP), which secured Mrs. May her majority in Parliament.
It has already been branded a "bribe" with Shadow Foreign Secretary Emily Thornberry describing the agreement with the DUP as a "grubby deal."
Theresa May to a nurse who hasn't had a pay rise in 8yrs: "there's no magic money tree"— Fiona Rutherford (@Fi_Rutherford) 26 June 2017
May to DUP: Here's £1.5 billion so I can keep my job
The revelation about the extra £1 billion (US$1.3 billion) for Northern Ireland was greeted with anger and resentment in other parts of the United Kingdom.
Welsh First Minister Carwyn Jones said it was a "straight bung" and "kills the idea of fair funding."
The Scottish National Party's (SNP) Ian Blackford said: "For years the Tories have been cutting budgets and services, but suddenly they have found a magic money tree to help them stay in power."
The DUP leader Arlene Foster was quick to tell Theresa May after the June 8 election that her party would support the Tories in principle.
But it took 18 days of negotiations, or horse trading. Now the party's 10 MPs have agreed to support the Conservatives in key votes in the House of Commons, especially when it comes to the Budget and Brexit.
In exchange, she has agreed to retain the triple lock on pensions, maintain defense spending and keep funding of agriculture in Northern Ireland at the same level for the rest of the current Parliament.
Labour warned it would undermine the Good Friday Agreement, which relies on the governments in London and Dublin acting as neutral third parties.
But Mrs. May, who called the snap election in April, but disastrously miscalculated, said she had a duty to form a government.
She said it would "enable us to work together in the interests of the whole United Kingdom, give us the certainty we require as we embark on our departure from the European Union, and help us build a stronger and fairer society at home."
Theresa May found her magic money tree, shaking it for £1bn to bung the DUP and survive as PM. Hers is the most subsidised job in the UK— Kevin Maguire (@Kevin_Maguire) 26 June 2017
The Northern Ireland Executive has been deadlocked since January with the DUP refusing to accept Sinn Fein's demands that their leader Arlene Foster should resign.
She has been blamed by Sinn Fein for a bungled Renewable Heat Scheme, which ended up costing taxpayers £400 million (US$510 million) more than it should have done.
Back in 2016 Democratic Unionist Party MP Jim Shannon was ordered to pay back almost £14,000 (US$17,800) in expenses by a Westminster watchdog. But the Independent Parliamentary Standards Authority said there was no fraud involved.
"What we have seen with Theresa May is her throwing money at the DUP in an attempt to prop up her government and continue running the country," Westminster expert Shehab Khan told Sputnik.
"The deal is going to be incredibly strenuous and she is going to find it very difficult to make sure the DUP toe the party line but what she has done is throw a lot of money at the DUP in the hope that they will side with her for the next five years," he added.
Theresa May and the dirtiest dirty dirty deal ever— ARTIST TAXI DRIVER (@chunkymark) 27 June 2017
£100M for every DUP vote but not 7 pence for a child's breakfastpic.twitter.com/CGZ3eVaean
"The Tories don't really have an alternative. If they're not making this deal with the DUP they have to run a minority government, which isn't going to work. The alternative is calling a general election and if they do that, Jeremy Corbyn will most likely win so they're not going to want to do that," he told Sputnik.
"Many people have raised questions about the DUP's spending in Northern Ireland, but Theresa May has thrown something like £100 million (US$127 million) per MP, which is a huge amount of money," Mr Khan said.
Theresa May's government signs deal with DUP, creating a weak & wobbly arrangement from the party that promised strong & stable leadership.— Brian Klaas (@brianklaas) 26 June 2017