I've had a look at the possible enlistment date for Thomas Pearsall Junior. It's a bit before my period, so my initial estimate wasn't all that close - I have narrowed down the date with the help of a few other service records (I had used Silver War Badge roll dates previously, which were misleading) and the date of enlistment is most likely August 1888.
This would make Thomas 17 years old, which was the minimum age of enlistment (without enlisting as a boy soldier). In theory, he wasn't old enough to serve overseas at that age, however since his father was probably still serving with the 48th in Malta at that point, the proper age of overseas service of 19 was probably irrelevant.
The 48th had been in Malta since 1867, and moved on to India in 1872, eventually returning to Ireland in 1880. I suspect that Thomas probably stayed with his father's battalion until they returned.
Have you attempted to trace the family births and marriage through the overseas Birth, Marriages and Death records.
Steve.