The business of Sinn Fein members not attending Parliament is a separate issue - they were elected on the basis of a firm commitment not to attend, and wouldn't have been elected if they hadn't made that commitment.

They'd presumably argue that the expenses claimed by them were to enable them, to carry out duties on behalf of their constituents, both in Ireland and in other places - that doesn't mean that there might or might not be grounds to question the validity of the expenses, as with all MPs, but the fact of their not sitting in the House of Commons is not in itself a reason to assume the expenses were not valid.