Under the terms of the agreement, Northern Ireland will receive an extra £1.0 billion (1.1 billion euros or $1.3 billion) from the state over two years in exchange for the DUP supporting Ms May's Conservatives.
May shook hands with DUP leader Arlene Foster as she and other senior party figures arrived at Downing Street on Monday to finalise the pact.
Ian Blackford, leader of the Scottish National Party's 35 members of parliament, said Scotland should also get its "fair share" of funding and accused May of discovering a "magic money tree" after years of austerity cuts.
With her own party in the minority after a devastating June 8 snap election ended in a hung Parliament, May has been forced to rally other conservative factions to her side in an effort to maintain power in the coming weeks.
The left-wing Sinn Fein party said the deal undermines power-sharing talks between nationalists and unionists in Northern Ireland that have so far failed to reach an agreement.
The arrangement does not guarantee support on all legislation in the Commons, although it is expected the DUP will back the majority of the government's programme for the next two years after many of its more controversial policies were dropped.
"Austerity has failed. Cuts to vital public services must be halted right across the United Kingdom, not just in Northern Ireland".
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May-led Conservative Party failed to secure majority in the recently concluded snap election in United Kingdom, as they are short of eight seats to from the government.
McDowell was part of a group of church leaders who are urging Northern Ireland's politicians to strike a power sharing deal for the common good of everyone.
The DUP has also agreed to support the May government on legislation regards to Britain's exit from EU.
That agreement in 1998 helped end decades of bloodshed between Northern Ireland's Protestant and Catholic communities.
Gerry Adams, president of the Irish Catholic nationalist Sinn Fein party, said the deal provides a "blank check" for a Brexit that threatens peace in Northern Ireland.
LABOUR'S candidate at the general election has questioned the value of the Conservative's DUP deal for Thurrock residents.
Irish Foreign Minister Simon Coveney was in Belfast Monday as part of negotiations aimed at restoring a power-sharing alliance between the DUP and Sinn Fein almost four months after local elections in Northern Ireland.