O'Connell and fellow ASU member Dowd flew from Shannon Airport, County Clare, to Heathrow in early August 1974, under the pretence of looking for work in London.
O'Connell, as the bomb-maker of the group, was responsible for making the first devices the ASU detonated in their campaign, during the Guildford pub bombings on 5 October 1974.
It began on 8 March 1973 when an 11-person ASU (which included Irish republicans such as the Price Sisters and Gerry Kelly) bombed the Old Bailey.
Although the IRA achieved its objective, it was a tactical error to try to leave so quickly, as British security forces would be extra vigilant at escape routes from England to Ireland.
The following attack was on 18 August 1973 when two IRA firebombs exploded at Harrods Department store, causing some damage but no injuries or deaths.
Just four days later an IRA book bomb exploded at the Conservative Party Central Office in London, injuring several people but none seriously.
A few weeks later IRA bombs went off at King's Cross and Euston stations causing 13 injuries and wide spread damage, and panic in central London.
[13] In follow-up raids after the siege, police discovered crossword puzzles in Brian Keenan's handwriting and his fingerprints on a list of bomb parts.
Another common method was making either a time bomb or an incendiary device with a timer on it, which would then be planted inside a pub, club, hotel, or other site.
The gang also shot and killed several people, the most famous of whom was Guinness Book of Records founder Ross McWhirter in November 1975.
Six of the dead were British military personnel, one was a London police officer, one was a member of the bomb squad, and 11 were civilians.