During the initial stages of the Anglo-Norman conquest of Ireland, FitzHenry gained a reputation for bravery that bordered on foolhardiness. He first distinguished himself in the invasion of
Ossory along with his cousin
Robert de Barry, older brother of Giraldus Cambrensis in 1169. FitzHenry played a prominent role in this campaign led by
Diarmait mac Murchada, King of Leinster, against Domnall mac Gilla Pátraic,
King of Osraige, in
Laois. Following this campaign, FitzHenry was then part of his uncle Robert FitzStephen's expedition to aid
Domnall Mór Ua Briain, King of Thomond. In 1171, FitzHenry was part of the group of Anglo-Normans that broke the
Siege of Dublin by the attacking army led by
Ruaidrí Ua Conchobair, King of Connacht. In 1172, he was assigned to the garrison of
Hugh de Lacy by the king. In 1173, the return of
Richard de Clare, 2nd Earl of Pembroke, named Strongbow, to England threw Ireland into revolt. Meilyr was then in garrison at
Waterford, and made a rash sortie against the Irish. He pursued them into the woods, and was surrounded. But he cut a way through them with his sword, and arrived back at Waterford with three Irish axes in his horse and two on his shield. In 1174 he returned with his cousin, Raymond le Gros to Wales, but le Gros and FitzHenry both returned back to Ireland in 1174. On his return to Ireland, FitzHenry was granted a
cantred in Kildare around
Carbury, but was deprived of this grant by King John's representatives John de Lacy and Richard de Pec. Instead, the king was granted a densely forested region of Laois, which was seen as a more fitting place for him. In October 1175 he accompanied Raymond le Gros in his expedition against
Limerick, was second to swim over the
River Shannon, and with his cousin David stood the attack of the Irish until the rest of the army had crossed over. He was one of the band of
Geraldines who under Raymond met the new governor,
William FitzAldhelm, at Waterford, and at once incurred his jealousy. Hugh de Lacy, the next Justice, took away Meilyr's Kildare estate, but gave him
Leix in exchange, a marcher district. In 1182 de Lacy again became Chief Justice, built a castle on Meilyr's Leix estate at
Timahoe, and gave him his niece as a wife. It seems probable that Meilyr had already been married, but he hitherto had no legitimate children. This
childlessness was, in Giraldus's opinion, God's punishment to him for the want of respect to the church. ==Under King John==