'Tar a Thighearna' was composed by Ruth Cunningham. The text in Gaelic is translated, "Come, Lord, come thou Being." Ruth and Ana Hernandez recorded it on Blessed By Light, one of their albums as the duo Harc.
Sheet music can be found in Singing In Community, our latest songbook published by Augsburg Fortress.
Here's a video of Rachel Kroh leading 'Tar A Thighearna' at Union Seminary in September 2015:
Here's a lovely recording of the song by Ana and Ruth.
Here's a link to a recording of Emily Scott singing the song and then making some suggestions for how to lead it paperlessly, from the resources Emily has been compiling for the song leaders at St. Lydia's in Brooklyn.