The Dundalk woman was acquitted of a second charge of attempting to finance terrorism by trying to send €800 to a known terrorist.
Smith kept her head down as the guilty verdict was delivered and began crying after the court rose.
Smith was granted bail and is due to be sentenced on July 11.
The verdicts were delivered by Mr Justice Tony Hunt this morning at the Special Criminal Court in Dublin following a nine-week trial.
It was the prosecution's case that Smith (40) made hijrah, or emigrated, to IS territory in 2015 to provide them support after establishing a caliphate.
In her defence she said she followed a religious obligation and denied aiding the terror group.
In relation to the membership charge, Mr Justice Hunt said that the conduct compelled by religious interpretation or belief pertaining to alleged membership cannot be justified or validated by conduct that would otherwise be unlawful.
The court said her interest in a particular interpretation of Islam was her choice and not due to any other influence in Ireland.
Evidence had been given of ISIS members and affiliates she had spoken with online prior to travelling to Syria.
Mr Justice Hunt said that online messages calling for secrecy and discretion were "telling" and questioned why this was needed if it was just an innocent group discussing Islam.
This included messages about the deadly Tunisian terror attack on a beach in 2015.
Under a post of the article Smith wrote: "Can't wait to hear the full story. Tourism bye bye hhh", which the court took to mean "Hahaha".
The judge said membership is not established through "fighting talk" or holding opinions. He said she did however travel to Syria in 2015 having conducted extensive research on "what and who awaited her".
Ms Smith, the court said, "knew full well" the techniques and views of those who enforced Sharia Law where she wanted to live. It was a group involved in the "most extreme and terrifying acts of violence" and left no room for pleas of naivety or ignorance.
It found that she "expressly" made Bay'ah, or gave allegiance, to Al-Baghdadi and his caliphate and that she took up membership of ISIS when she crossed the border into Syria in 2015.
Referencing interviews she gave after her arrest in which she talked about "running with the crowd", the judge said these were closer to buyer's remorse rather than a denial.
They said there was no doubt that Lisa Smith was a membership of ISIS and that the prosecution had made its case on the evidence presented.
In relation to the second charge, the non-jury court said there was "no doubt" that at the time of the attempted money transfer the intended recipient, John Georgelas, was a member of ISIS involved in propaganda and fighting.
However, he added it could not be ruled out beyond a reasonable doubt that the intention was charitable or for humanitarian reasons, instead of financing terrorism.
Mr Justice Hunt said they were not satisfied the prosecution had met its case in relation to the fundraising charge and directed a not guilty verdict.
Prosecutor Sean Gillane SC had told the trial the defendant understood exactly what she was doing when she travelled to Syria in October 2015.
He said that, while IS needed fighters, it also needed people who could give sustenance and vitality to the group in the achievements of its aims and to answer al-Baghdadi's call. This included her husband Sajid Aslam who was a fighter for the group.
Mr Gillane said the accused subscribed to the reciprocity of allegiance and protection, and in that sense "enveloped herself" in the black flag of IS.
The court was told she remained in the territory and married there, and that her movements "mirrored the retrenchment of the group" from when it lost territory.
"Far from moving away or running away from them, she was running with them," Mr Gillane told the Special Criminal Court.
However, it was Lisa Smith's defence that she travelled to IS territory as part of a religious obligation, that she never even picked up a weapon, and that she was "led astray" as to what IS stood for. In her garda interviews she said she didn't know what ISIS was and that she only knew it as IS.
Her senior counsel Michael O'Higgins said that the garda interviews were conducted on an unfair basis, that evidence from an expert witness on Middle Eastern conflicts was troubling, and that the prosecution falls short to prove the case against his client.
Throughout the trial evidence was heard of how Lisa Smith, who served with the Defence Forces for ten years, conversed with a "gallery of confederates" online before travelling to Syria.
This included Georgelas who was regarded as her mentor and educated her on Islam.
Others were Robert "Musa" Cerantonio, an extremist preacher from Australia, and Melbourne man Isa Kocuglu who was convicted of fundraising for Gerogelas.
She had pleaded not guilty to being a member of the unlawful terrorist group IS, also known as ISIS, between October 28, 2015, and December 1, 2019.
She had also denied trying to finance terrorism by attempting to provide €800 in assistance, via a Western Union money transfer, to a named man on May 6, 2015, for the benefit of the same terror organisation.