The 21-year-old Republic of Ireland Under-21 international, who can play anywhere on the left flank, had a loan spell at St Mirren last season.
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The Republic of Ireland Under-21 international played under new U's boss Aidy Boothroyd at Vicarage Road.
McCall is expected to be initially involved with the Ireland under-21 side for a season.
Former Republic of Ireland Under-21 international Sammon began his career with University College Dublin before moving to Derry City.
"He has played with Alan Judge and Cillian Sheridan in the Republic of Ireland Under-21 side, " said Argyle boss Paul Sturrock.
Former Northern Ireland Under-21 midfielder Conall Murtagh, 23, has arrived from Wrexham.
The 23-year-old former Republic of Ireland Under-21 international has had loan spells with Accrington Stanley, Gillingham, Bury and Hereford United while at the Valley.
Republic of Ireland Under-21 international defender Wilson, 21, made his debut for Pompey in a Carling Cup tie against Chelsea last September and has made three Premier League appearances.
The Republic of Ireland Under-21 international sent a magnificent reverse pass into the inside-right channel for James McArthur to run on to but the midfielder's shot was too weak and was held by Olejnik.
Meanwhile Linfield are also understood to be in talks with former Northern Ireland midfielder Philip Mulryne and ex-Cliftonville and Lisburn Distillery frontman John Martin while Northern Ireland Under-21 striker Paul Carvill is also a target.
On top of the loan signings of Manchester United duo Sam Hewson and Fabian Brandy and Liverpool pair Peter Gulacsi and Godwin Antwi, Turner also brought in Cardiff City's Republic of Ireland Under-21 international defender Darren Dennehy last week.
Australia-born Lowry, who represented the Republic of Ireland at Under-21 level, recently decided to play international football down under.
The brother of Burnley's Michael, Duff has been capped at under-21 level for Northern Ireland and becomes Bradford's sixth new signing of the summer.
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