Irish terriers, when they have gained maturity, are notable, not alone for their courage, fidelity, and capacity for love, but for their cool- headedness and power of self-control and restraint.

They are less easily excited off their balance; they can recognize and obey their master's voice in the scuffle and rage of battle; and they never fly into nervous hysterics such as are common, say, with fox-terriers.
Michael possessed no trace of hysteria, though he was more temperamentally excitable and explosive than his blood-brother Jerry, while his father and mother were a sedate old couple indeed compared with him.

Far more than mature Jerry, was mature Michael playful and rowdyish.

His ebullient spirits were always on tap to spill over on the slightest provocation, and, as he was afterwards to demonstrate, he could weary a puppy with play.

In short, Michael was a merry soul.
"Soul" is used advisedly.