Very little is known about Laȝamon beyond what there is in the ‘prologue’ to his poem. I’ve written an introduction to him which you can find in the articles, essays, reviews section of this site.
You can hear the original prologue in its early middle English here:
There is no reason to believe Laȝamon went to Ireland, or knew Gerald of Wales, but A Presentment of Englishry plays with the idea. Gerald was certainly there with John, ‘Not yet a King’ and the poem is based on his version of events.
The three long stories in A Presentment of Englishry are broken by ‘Interludes’. The first of these tells three stories taken from Gerald of Wales.