The controversial pact with the Northern Irish Democratic Unionist Party comes after Ms May ended up losing her parliamentary majority in an election earlier this month that she had called to boost her support for Brexit talks. A party needs 326 seats in order to have a majority in the UK.
As part of the deal, 10 lawmakers from the DUP will also support May's Conservative party in the Parliament including voting for the Queen's Speech, which sets the legislative program for the country which was delayed from the original date on June 19.
The massive injection of funds her government pledged in exchange for the deal struck with the Democratic Unionist Party (DUP) angered the leaders of Scotland and Wales, as well as Northern Ireland's other parties.
"This Tory-DUP deal is clearly not in the national interest but in May's party's interest to help her cling to power", Labour leader Jeremy Corbyn said.
Labour's Kevin Brennan said that the continued provision of Short money, along with the extra funding, amounted to "double bubble for her friends in the DUP".
Meanwhile, the Scottish National Party's Westminster Leader, Ian Blackford, said Scotland was being offered "little more than scraps from the table" under the deal.
As part of the deal, signed by Gavin Williamson for the Conservatives and Sir Jeffrey Donaldson for the DUP, there will be no downgrade to the pensions triple lock and the winter fuel payments will remain.
The Defence Secretary has defended a £1.5bn deal with the DUP amid a mounting backlash, saying it will protect the Government from being "ambushed" on Brexit and other big issues.
The cash will go to the Northern Ireland executive if the devolved institutions are restored by the deadline of June 29.
Mr Adams said any extra money for Northern Ireland is a good thing, adding: "We may be able to say well done Arlene, when we have the Executive in place".
Sinn Fein said last week that "time was running out" given the lack of knowledge about the impact of any Conservative/DUP deal. The party has long played a central role in Northern Ireland and now stands to gain more influence in Parliament as well.
Meanwhile, Northern Irish politicians have just three days to agree on a power-sharing agreement in Stormont or face direct rule from London.
"I think the most important opportunity that exists for us here, past this election, is the mainstreaming of the DUP in United Kingdom politics", O'Doherty said.