Former Shelbourne man McAllister was one of the key performers for Pats last season, missing just four league games for the Saints and also helping them to the Setanta Cup final and the semi-finals of the FAI Cup.
That has sparked cross-channel interest in the 22-year-old and the midfielder arrived back in Dublin last night after a trial at Sheffield United.
The Blades were impressed with McAllister and will consider making him an offer next week, but a number of other Championship sides are interested in the ex-Drogheda United player.
"There is quite a bit of interest in David. We will see what comes of it, but he has a good chance of getting a move to England," said a source close to the player.
Pats boss Mahon was delighted with McAllister's form last season and he says the club won't stand in his way if he does move to the UK, but Mahon has not ruled out the prospect of the player re-signing for the Saints.
"According to the reports we had, David did well in Sheffield last week so we'll see what comes of it. But we don't want to lose him," said Mahon.
"David has said that if he doesn't get a move to England he would like to re-sign with us. He has said that he feels a loyalty to St Pats, so if he does end up staying in Ireland I'd be confident he will be back with St Pats."
Mahon has begun talks on new contracts for some of last season's squad at Richmond Park and the hope is that key players like Damian Lynch, Conor Kenna and Stuart Byrne will re-sign. Byrne had effectively decided to retire before he signed for Pats at the start of the 2009 season but the former Shelbourne man has been revitalised.
"I would like to see Stuart stay with us as he was a very important player. We have made him an offer," he added.
But the trio of Byrne, Coughlan and Faherty have played their last games for the Saints.
Byrne started well for Pats last season with form that earned him a call-up to the Ireland U23 squad but an ankle injury disrupted his season.