I suspect that there were two ways that the Irish settled in Texas.
One was through landgrants from the Mexican Government through recruiters or Empresarios who recruited Irish Catholics in Ireland and in New York City and possibly other cities in the United States. This is how my HART ancestors arrived in South Texas about 1929.
The Mexican Government recruited these Irish Catholics for two reasons.First because they could not get Mexican citizens to settle in South Texas because of the Indian threat (primarily Comanche but, some other tribes as well).The Irish were to become buffers between the hostile Indians and Mexico proper.
The second reason was also to use the Irish as a buffer but, this time as a buffer between the Anglos who were pouring into Texas accross from Louisiana under the auspices of Austin and his colonies.
The Mexicans believed that the Irish, being Catholic and being generally despised by the Anglo Americans would side with the Mexicans because of their common religion.
Unfortunately for Mexico, that did not work out as the Government planned.Many Irish Catholics revolted against Mexico.My great-great-great-great Grandfather's brother (Timothy Hart, an Irish Catholic) signed the Texas Declaration of Independence at Goliad.
The second way Irish came into Texas was directly from the United States.There were likely some Irish Catholics who had previously settled in the United States who joined the Austin Colony. Others may havjust traveled to Texas to take advantage of the cheap and sometimes free land that was available there.
There are many books regarding the Irish settlement of Texas.Some of the more important ones are:
"The Texas Irish Empresarios and Their Colonies; Refugio and San Patricio" By: Father William H. Oberste (Probably the definitive history of early Irish settlement)
"The Forgotten Colony; San Patricio de Hibernia" by Rachel Bluntzer Hebert
Shamrock and Cactus; The Story of the Catholic Heroes of Texas Independence" By: W.M. Ryan
Ireland and Irish Emigration to the New World; from 1815 to the Famine" By: William Forbes Adams, PhD.
I have excerpts from these books and will try to locate your ancestors.