A breach of the federal compact will be no light matter.
The constitution again, as one would expect under a federal scheme, provides for the enforcement of the compact.
In the case of Ireland this is manifest.

The royal veto,[21] the power of the Courts, and ultimately of the Privy Council, to pronounce on the constitutionality of any Irish Act, and treat it as void if it is in excess of the authority bestowed upon the Irish legislature, the provisions for the legal determination of constitutional questions,[22] the arrangements as to the payment of the Irish customs into the Imperial Exchequer, the special and very anomalous position of the Exchequer Judges, are all attempts, whatever be their worth, to restrain the Irish legislature and government, or in effect the Irish people, from the undue assertion of State rights.
Restraints again are placed on the unconstitutional action of the Imperial or Federal Parliament.

They are less obvious, but at least as real and effectual as the safeguards against the breach of the constitution by the Irish government or legislature.

They are all summed up in the presence of the Irish representatives at Westminster.

The only legitimate reason, if legitimate reason there be, for their presence is the guardianship of Irish rights under the constitution.