[8] Having reached the level of Senior Scientist, he was offered a long-term NASA job, but wanted to return to Ireland, and when his wife was offered a post at University College Dublin (UCD) in 2005, they decided to move back together, even at significant reduction in pay, and he secured an initial job teaching Space Science at UCD before receiving an opportunity to head the Solar Physics & Space Weather Research Group[10] at Trinity College Dublin (TCD) from the beginning of 2006.
[15] Shortly after returning to Ireland, he was asked to take a key role in twin NASA solar observation flights back in the US, designed to study the massively higher temperature of the Sun's corona compared to its actual upper layers.
[16] In 2009 he and his team participated, with the Royal Observatory of Belgium, in a satellite project, PROBA-2, to study solar storms; they wrote software for two elements.
[20] Also in 2015, he led the building of a magnetometer network by TCD and the Dublin Institute for Advanced Studies, capable of detecting solar storm activity.
[24] He has stated that he would like to open up the observatory campus, which is near a cycling and walking "greenway" along the Royal Canal of Ireland and the River Tolka, adding a coffee shop and growing visitor numbers from 5,000 to 50,000, making it s significant tourist and cultural attraction for West Dublin.
An initial simple vertical antenna site was made, and later elaborated, and Gallagher remains the director of what became the Rosse Solar Terrestrial Observatory.
[27] He agreed the principle with Lord Rosse, then secured 50 thousand euro from Dermot Desmond, who then put him in touch with Denis O'Brien, who called and after discussion also wired a substantial contribution.
"[2] The Irish LOFAR site was launched in July 2017, and its aims included monitoring of solar activity, light waves from the early history of the universe, and potential signals from intelligent extraterrestrial sources.
[28] The I-LOFAR telescope has 3,000 antennae and 55 km of cabling, and provides opportunities for a range of PhD and post-doctoral students, as well as lecturers and professors, to advance their work.
[34][35] As Director of I-LOFAR, Gallagher also co-presented a programme, "13 Billion Miles from Birr" on RTE TV, in 2017, to mark its launch and planned work.
His most-cited first-author paper is "Rapid acceleration of a coronal mass ejection in the low corona and implications for propagation" in the Astrophysical Journal Letters (2003).
They kept in touch while pursuing advanced studies in Belfast and elsewhere, living near Washington, D.C., for part of their time in the US, and moving back to Ireland together when Teeling received a job offer.