Complex sentence: There is reason to suspect that the Leinsterman's success stemmed from assistance received from Mac Lochlainn, since the latter is recorded in the same source to have laid siege to the town within the year.
Split into simpler sentences: There is reason to suspect that the Leinsterman's success stemmed from assistance received from Mac Lochlainn. The latter, for example, is recorded in the same source to have laid siege to the town within the year.