Other trade unions are also being asked to invest in the 20m film Connolly on the basis that the Scottish-born socialist established the Irish trade union movement in the early 1900s.

Connolly features Peter Mullan of Trainspotting fame as Connolly, and Patrick Bergin who starred alongside Julia Roberts in Sleeping with the Enemy, as 'Big' Jim Larkin, who set up the rival Federated Workers Union of Ireland. Susan Lynch will play Connolly's wife, Lillie.

Adrian Dunbar, who starred in Hear My Song, will direct the film, which tells the story of Connolly's life and exploits through the eyes of his daughter, Nora.

The general-secretary of Siptu, Joe O'Flynn, said that the union has agreed in principle to invest in the film but will not make any decision on the level of investment until Siptu sees how much the producers, Rascal Films, have secured from other sources.

The union is taking outside financial advice on the investment. "Siptu has no experience in the movie business and while it might be a modest investment for the producers, it will be a significant sum for us, " said O'Flynn.

"While we are keen to support a mainstream movie which will highlight the trade unions' contribution to the birth of the Irish free state, we also have to be prudent."

Siptu, which employs over 300 staff, has experienced financial troubles of late and last year had to ask its staff to make increased contributions to their pension. "Of course, the investment could also pay off" said O'Flynn.

Arts minister John O'Donoghue has also come under considerable pressure to directly invest 5m in the film on the basis that it will be the only Irish-made film about the 1916 Rising.

It is understood the film will also take a traditionalist view of 1916, similar to the approach taken by Ken Loach in his controversial film The Wind that Shakes the Barley, which was criticised in some quarters as being antiBritish.

A spokesman for O'Donoghue said that his department cannot make a direct investment in any film.

Any such application for funding must be made to the Irish Film Board, which is the independent state agency funded by the department, he said. The spokesman added that the total allocation to the Irish Film Board for 2006 was over 17m . . . up 18.5% on the 2005 allocation.

Tax breaks for investors in Irish-made films had also been significantly improved, the spokesman said. Previously, a production company could raise a maximum of 15m in funding through the tax-breaks scheme but this has now been increased to 35m.

Producers Rascal Films have set up a charitable foundation, the James Connolly Foundation for Educational Equality, which will be used as a tax-free vehicle to fund the film.

Individuals or groups can subscribe to the foundation which "may be be eligible for tax relief", according to the producers.

The foundation will then invest in Connolly with any profit going back into the foundation.

Shooting is on schedule to begin this autumn and the producers hope to have the film released next year.