The on-the-run killer gave himself up yesterday after 12 years hiding from justice.
Gallagher, who killed Twin Towns mother and daughter Anne and Annie Gillespie in the grounds of Sligo Hospital in 1988, was living in Strabane.
Gallagher, from Lifford, killed Anne, 22 and her mother Annie, 51, after learning how Anne planned to dump him.
But he was found not guilty of murder and sentenced to an indefinite life at the Central Mental Hospital in Dublin from where he escaped in 2000.
Sources say Gallagher walked into the facility yesterday, but only after carefully planning a legal case which will declare him sane – and allow him to walk free.
His release is a matter for Justice Minister Alan Shatter.
But we understand that Gallagher, who is now married with children, wants to be able to visit his mother in Lifford without being detained by Gardai.
By giving himself up, he will be able to do that on his release.
That may come within weeks, said one source, as Gallagher plans a High Court bid “almost immediately” to have himself declared sane again.
If he wins the case – and he will argue that he now leads a normal life in Tyrone – he would be freed.
Members of the Gillespie family are said to be “devastated” at Gallagher’s legal challenge.
“They’ve always believed he was guilty of two murders and that they never got justice,” said one today.
How donegaldaily.com broke the story last night: